How did Galaxies Form

After recombination there were obviously pockets of density enhancements around which material accumulated

The exact mechanism for this is unclear and most of the mass is probably darkmatter. Gravitational coalescence of this dark material then built the galaxies we observe today. Below is an N-body supercomputer simulation of this process.

These eventually grow to form galaxies. Unfortunately, there is a rather wide range of different galaxy types, shapes and densities which means the formation process was not simple. Also most all galaxies today are embedded in some larger scale structure. The formation of these structures is unclear but here are a couple of possibilities:

Structure formation could have either occurred from fragmentation of very large regions into smaller regions or from the gravitational coalesence of sub-units into successively large structures. Both formation scenarios lead to a highly clustered Universe with structure on many different size scales.

Individual Galaxy Types:

So many questions about galaxy formation and evolution certainly remain. Among them are:

For our purposes a galaxy is just a large collection of gas which is gravitationally bound. This gas eventually clumps to make stars. Without the formation of stars in galaxies, the Universe would have never made elements heavier than Helium.

Hence, the formation of planets with life depends upon the initial formation of galaxies, which is largely an unknown process.

Galaxies represent essentially collection sites for the material made in the early Universe to coalesce into big balls of gas which essentially ignite the fires of fusion in their centers thus becoming stars.

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