Accelerated introduction to large-scale structure

Here is a list of resources, mainly conceptual, for understanding the structure of the universe as revealed by modern observations:

Now think about the last video above carefully. A galaxy cluster is a set of masses attracting each other gravitationally. Consider a volume of space “riding along” in the expansion, and let it have an excess of density compared with the average of the Universe. If there is enough excess mass within that volume, it can slow, or even reverse, the expansion within that limited volume. This concentrates the matter and leads to the formation of galaxies etc. But current models indicate that excess mass beyond what is visible in ordinary matter is needed to seed the formation of galaxies. Otherwise, they would not have collapsed together in the time since the Big Bang.

So dark matter seems to seed the formation of galaxy clusters and superclusters and lead the the large scale structure of the Universe, with a filamentary structure. The type of structures that are seen are consistent with cold dark matter: matter which is thermostatisically cooler than the other possible candidates, such as “warm” neutrinos, which are known to exist in small amounts.

What then results is a set of contracting "halos" of dark matter and matter embedded in the expanding universe.

Dark matter haloes in expansion
A small group of halos in an expanding universe observed from a point O. The red arrows show the halos contracting as they are embedded in the cosmic expansion (black arrows).