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Dark energy is a term used to describe an unknown reason for why the universe is growing faster year by year (well, billion year by billion year). Ordinary (aka Newtonian) physics suggests the universe should be growing slower year by year until it starts shrinking. Galaxies are attracted to each other by gravity.

Galaxies behave as if they're much heavier than the stars, planets, black holes, etc. that can be seen. The most popular theories are that galaxies contain lots of invisible (dark) matter that makes them heavier.