discuss the merits and demerits of various theories of the formation of galaxies.

There are mainly 2 kind of theories; top-down theories and bottom-down theories. The first ones claim that there were big masses of nebula that started breaking down into galaxies; the second claims that initially, small lumps of matter coalesced into bigger structures and finally galaxies. The first one is very elegant and is very consistent with what we know about the formation of other structures, nevertheless it lack experimental data that corroborate it. The second has been more successful with that, but there are still many theoretical questions that cannot be explained. Both are not equipped to handle dark energy well.

Answermake them get very hot and over time wear them out;