For reference:
Referring to a question I posted in Feb. 2015:
Could Hyper-Massive Black Holes be due to Dark Matter in the Early Universe?
User10851 answered, and I replied. He then replied to my comment with:
"at 1 Gyr old the average density of the universe was less than 1000 times what it is now"
Which does not make sense to me, and leads to my question:
How can the universe's density increase over time? To me, if the universe is expanding, the density is decreasing unless new particles are popping into existence at a greater rate than the expansion (which I don't believe).
The reason this is important to me is that the density fact seemed to be the stop-fact for that theory, but I don't believe it's accurate. At the same time, I don't like questioning experienced physicists.
I believe that if Dark Matter truly does not interact with electromagnetic fields, and is weakly-interacting, but remains affected by gravity, expansion, and other forces, then the quantity of dark matter would have been significantly greater (perhaps exponentially) after the big bang, and these early black holes that are larger than they are supposed to be could be explained by their gobbling up of massive quantities of dark matter in the early universe. Since Thompson Scattering and Eddington Luminosity do not affect dark matter as they do other forms of energy and matter, this could be an explanation for larger-than-expected black holes 900 million years after the universe's birth, which in the end leaves us with the quantity of dark matter we have today (about 5.75 times that of visible matter).
Best Answer
It very much sounds like you misread the comment. "The average density of the universe was less than 1000 times what it is now" means that it was more than it is now, but less than 1000 times so. So, say 200 times so.
The density of the universe has definitely decreased over its lifetime. The dark energy density remains constant (in the simplest models, and nearly constant over the lifetime so far regardless), while the other components have decreased: