Most astronomers believe that all of the galaxies were formed around the same time, 10 to 20 billion years ago.
They probably came into existence only a few hundred million to a billion years after the big bang.
The nucleus of a galaxy is a powerful source of energy.
It is commonly thought that some galaxies, including the Milky Way, may each have a black hole at their centers.
A galaxy will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life.
This matter is mostly hydrogen and helium.
