1. Dark Matter
In the 1970s, using spectral analysis, it was discovered that spiral galaxies were not rotating like they looked like they should be.
If you have a system where most of the mass is in the center, like the solar system with the Sun having 99.8% of the mass, the closer a planet is, the faster it moves.

2. Spiral galaxies look like most of the mass is near the center in the nucleus, so nearer stars to the center should rotate faster.

3. Astronomers found that stars near the center of spiral galaxies were rotating slower than stars farther out, implying that most of the mass was not in the bright center.

4. There had to be some other invisible mass changing the rotation speed of stars. This got the name “Dark Matter”.
Several explanations for this “Dark Matter” have come forward.
Neutrinos
MACHOs
WIMPs/Sparticles
Dark Energy

5. Neutrinos are subatomic particles given off during nuclear fusion
Neutrinos are subatomic particles given off during nuclear fusion. They don’t react with much of anything, and thus are nearly impossible to detect. Right now buzillions of them are traveling through you, and the Earth from the Sun. If they weigh something, they could account for the missing mass.

6. MACHOs are massive compact halo objects, implying they are objects with mass that don’t give off light (in the halo or outskirts of a galaxy). These things could be planets, black holes, or neutron stars.

7. WIMPS are weakly interacting massive particles, meaning they are massive subatomic particles that don’t react with other matter. They have yet to be “discovered”. String theory predicts a similar particle called a “sparticle”.

8. Dark Energy is an undefined source of matter/energy that is causing the entire Universe to expand faster than it should be. There are no explanations of what it is or what causes it.