Question: how has the universe changed over the years?

Monique Henson answered on 9 Nov 2015:

Mainly it’s a bit more clumpy and there are a lot more stars around. When the Universe was young, matter (the stuff that makes up everything around us) would have been spread out much more evenly across the Universe. There would have been some bits with more and some bits with less, but overall it was fairly smooth. However all of this matter is subject to a force – gravity. Gravity makes the regions where there’s a bit more stuff collapse and get smaller and smaller. As these bits collapse, they get hotter and they start to emit light. When they get really small, the gas in these regions stops gravity pulling it in, and it stops collapsing. If the gas is hot enough that nuclear reactions can begin, a star forms. This happens all over the Universe at lots of different places, so stars start forming all over the place. Not just stars either – as everything starts collapsing, you get rocky objects forming (planets, asteroids etc.), galaxies (groups of stars and gas) and even black holes.

Lots of scientists try to model this using computers to try and understand it better. You can see a video from one of these models here (from the Illustris simulations).