Dark matter galaxy, not many stars:

Originally posted by twhitehead
Explain your reasoning behind this because at face value it doesn't make any sense.

[b] but Hawking radiation would ensure a very short lifetime
How short. Give us a figure for black holes the size of the sun or the size of Jupiter. But first justify your claim that they must be small.[/b]

They must be small because if they were large they would be attracting matter around individual black holes and if so there would be visible effects going on, local swirling of stars and such that we see in large ones.

Here is a wiki about small ones:

https://en.wikipedia.org/wiki/Micro_black_hole

In our age of the universe it states a black hole of mass 10^12 kg would be evaporating right now and smaller ones would already be gone, evaporated.

But when they evaporate, they do it with a burst of particles which would for sure be able to be seen with our instruments, gamma ray, X-ray, IR, UV and probably even radio waves.

That would result in a totally different kind of radiation pattern seen in the universe.

Originally posted by sonhouse
They must be small because if they were large they would be attracting matter around individual black holes and if so there would be visible effects going on, local swirling of stars and such that we see in large ones.

A black hole that was 19 times the mass of the sun would behave roughly the same as a star 19 times the mass of the sun, except without the bright light. You would not see them and you would not be able to detect them easily.

Here is a wiki about small ones:
https://en.wikipedia.org/wiki/Micro_black_hole
In our age of the universe it states a black hole of mass 10^12 kg would be evaporating right now and smaller ones would already be gone, evaporated.
You mean that if they started off that size at the start of the universe.

But when they evaporate, they do it with a burst of particles which would for sure be able to be seen with our instruments, gamma ray, X-ray, IR, UV and probably even radio waves.
Evidence please. Give actual figures as to how bright they would be.

That would result in a totally different kind of radiation pattern seen in the universe.
Actually it was at one point believed that that was what gamaray bursts were, but it is now believed that gamaray bursts are actually black hole creation events.
The fact is that there are a lot of black hole out there that we know about.
What we don't know is exactly how many and what mass distribution they are and whether that accounts for dark matter.
Certainly you are quite badly mistaken about how easily detectable they are or about what survey have been done to determine how many there are. That there are enough of them to account for dark matter is still an open possibility.

Originally posted by sonhouse
But when they evaporate, they do it with a burst of particles which would for sure be able to be seen with our instruments, gamma ray, X-ray, IR, UV and probably even radio waves.

You didn't even bother to read the Wiki page you cited.

In optimistic circumstances, the Fermi Gamma-ray Space Telescope satellite, launched in June 2008, might detect experimental evidence for evaporation of nearby black holes by observing gamma ray bursts.

Originally posted by sonhouse
In our age of the universe it states a black hole of mass 10^12 kg would be evaporating right now and smaller ones would already be gone, evaporated.

Black hole from the mass of the moon (4.5 × 10^22 kg )upwards, would grow over time not shrink.

Originally posted by twhitehead
A black hole that was 19 times the mass of the sun would behave roughly the same as a star 19 times the mass of the sun, except without the bright light. You would not see them and you would not be able to detect them easily.

[b]Here is a wiki about small ones:
https://en.wikipedia.org/wiki/Micro_black_hole
In our age of the universe it states a blac ...[text shortened]... here are. That there are enough of them to account for dark matter is still an open possibility.

Well maybe, but the gravitational lensing point did occur to me as well. If there are all these ~30 solar mass black holes everywhere why haven't we seen them?

Originally posted by DeepThought
Well maybe, but the gravitational lensing point did occur to me as well. If there are all these ~30 solar mass black holes everywhere why haven't we seen them?

With small ones they have to line up with a more distant object and considering the line of sight to another distant object statistically is vanishingly small, I would be surprised at ANY microlensing events. They do happen of course but most I heard about were from stars lensing more distant objects not black holes. The stars would be a lot bigger than a small mass black hole so statistically geographically much more likely to be in line of sight of some distant object.

Originally posted by DeepThought
Well maybe, but the gravitational lensing point did occur to me as well. If there are all these ~30 solar mass black holes everywhere why haven't we seen them?

The gravitational lensing technique is certainly a valid method for searching for black holes. And I believe there are surveys planned or underway to do just that. What I do not know is whether they have been done to the point of ruling out large numbers of black holes, and I know that sonhouse does not know this either.
I know that these surveys are also done in order to look for interstellar planets - which are also a form of 'dark matter' whose quantity we have not yet put a number on as far as I know. (its been a few years since I did a course on the subject so there may be new results that do narrow it down).

Originally posted by sonhouse
With small ones they have to line up with a more distant object and considering the line of sight to another distant object statistically is vanishingly small, I would be surprised at ANY microlensing events.

The secret is to monitor large numbers of stars at once.

They do happen of course but most I heard about were from stars lensing more distant objects not black holes.
But do you know of any thorough surveys that can put a figure on both star events and black hole events?

Some surveys are mentioned here, but I haven't gone to each to check the results:
https://en.wikipedia.org/wiki/Gravitational_microlensing#Search_collaborations

Originally posted by DeepThought
Well, ok., but this isn't proved, it's a theory with some evidence to support it and the particle based theories shouldn't be ruled out yet.

Okay, but we know black holes exist. Another form of dark matter is not known and is merely theory and nothing more. Why would anybody cling to a theory that only complicates things rather than put more effort into finding more black holes? The center of the galaxies must have huge black holes. I stand by my assertion that black holes are the dark matter.

Sometimes the more simple explanation is the best.

Originally posted by Metal Brain
Okay, but we know black holes exist. Another form of dark matter is not known and is merely theory and nothing more. Why would anybody cling to a theory that only complicates things rather than put more effort into finding more black holes? The center of the galaxies must have huge black holes. I stand by my assertion that black holes are the dark matter.

Sometimes the more simple explanation is the best.

Well, exploring one possibility does not rule out exploring the other. To my knowledge there have been two LIGO observations of merging black holes. The rate of black hole mergers gives us a handle on how many black hole binaries there are and so allows us to count black holes. There is also conventional astronomy. Particle based searches for dark matter candidates happen at LHC, but they'd be doing that even if no one had ever thought of the concept of dark matter. So there's no particular problem, or even excess resources expended, with following both leads.

Originally posted by Metal Brain
Okay, but we know black holes exist.

And we know of a number of possible other things that could reasonably explain dark matter. Its a question of following the evidence not guessing the in the dark.

The center of the galaxies must have huge black holes.
Many of them do, but dark matter is fairly evenly distributed throughout galaxies and also has more bulge than the disc shaped visible matter (I think, its been while, so I could be recalling wrong). It also has a number of other properties. To what extent those properties can be explained by black holes I am not sure. I suspect more research is required.
What I do know is that the black holes in the centres of galaxies isn't enough (we have actually measure the mass of the one in the milky way). I also know that guessing is not how science works. Science is based on evidence.

[edit]
Just checked, and yes, dark matter forms halo's not disks.

Originally posted by Metal Brain
...Why would anybody cling to a theory that only complicates things rather than put more effort into finding more black holes? .

what if there aren't enough black holes to find to explain dark matter?
Dark matter theory doesn't and isn't there to "complicate" things but explains the way galaxies rotate the way they do.
Just because dark matter merely could be one thing (black holes) doesn't rule out it being something else; we first need evidence to do that.
In science, you mustn't ever rule out any creditable theory until if or when there is creditable evidence against it and, until then, you should explore all the possible creditable theories until you have eliminated all but one of them via evidence and, whichever theory is left still standing, that is the theory you should assume to be the correct one or at least the most probable.
That is just how science works.

Originally posted by DeepThought
Well, exploring one possibility does not rule out exploring the other. To my knowledge there have been two LIGO observations of merging black holes. The rate of black hole mergers gives us a handle on how many black hole binaries there are and so allows us to count black holes. There is also conventional astronomy. Particle based searches for dark ma ...[text shortened]... So there's no particular problem, or even excess resources expended, with following both leads.

Sure, I'll agree with exploring all possibilities. I think black holes is the most likely though. There are decent explanations as to why the number of them is underestimated. I pointed out one on this forum before. Do you remember?

Originally posted by humy
what if there aren't enough black holes to find to explain dark matter?
Dark matter theory doesn't and isn't there to "complicate" things but explains the way galaxies rotate the way they do.
Just because dark matter merely could be one thing (black holes) doesn't rule out it being something else; we first need evidence to do that.
In scienc ...[text shortened]... uld assume to be the correct one or at least the most probable.
That is just how science works.

"What if there aren't enough black holes to find to explain dark matter?"

That has not been determined yet. I doubt that is the case.