From the book "Genesis and the Big Bang" by Gerald Schroeder.
"When the writers of the Cosmos series claimed that without the modern equipment available to those involved in cosmic research we would not suspect that the universe is expanding, that we would have no inkling that the universe probably expanded from a primordial state of high density, that is, we would not have discovered the phenomena of the Big Bang -- they were, of course, correct. Discovering the phenomena related to the original Big Bang required sophisticated radio waves and optical telescopes and all technology related to high-energy particle accelerators. These became available only in the last 50 years or so.
Now consider the writings of Nahmanide who lived from 1194 to 1270 C. E.
He writes, "At the briefest instant following creation all the matter of the universe was concentrated in a very small place, no larger than a grain of mustard. The matter at this time was so thin, so intangible, that it did not have real substance. It did have, however, a potential to gain substance and form and to become tangible matter. From the initial concentration of this intangible substance in its minute location, the substance expanded, expanding the universe as it did so. As the expansion progressed, a change in the substance occurred. This initially thin noncorporeal substance took on the tangible aspects of matter as we know it. From this initial act of creation, from this ethereally thin pseudosubstance, everything that has existed, or will ever exist, was, is, and will be formed."
So did God give him a revelation or was it just a really lucky guess?