Question

What effects does dark matter have on galaxies?

Answer 1

Dark matter is needed to explain observations. It is a hypothesis, yet unconfirmed.

Explanation:

If you look at matter in the universe, using telescopes, you can compute the mass of what you observe. But if you observe galaxies's motion, you find that that motion supposed more mass than what is seen. Hence the name, dark matter. 85% of matter is "dark". Hypothesis range from cold to hot dark matter, Wimp (Weakly Interacting Massive Particles) to MACHOS (Massive Compact Halo Objects.). It's a developing story. Subject of active research.