One of the first steps of the scientific method is coming up with a hypothesis, or just an educated guess or prediction about how/why something happens. The prediction must be tested by experiments or observation to see if that guess is right or wrong. For example, a famous hypothesis is that the speed at which an object falls to the ground does not depend on that object's mass. This hypothesis predicts that if you dropped a hammer and a feather from the same height at the same time in vacuum (without air resistance), then they should hit the ground at the same time. To test this, an astronaut actually tried dropping a hammer and a feather while standing on the Moon (which does not have air), and he showed that the hammer and the feather really hit the ground at the same time. This is evidence that the hypothesis was true.
Even though astronomers do not understand exactly what dark matter is, over the years they have had lots of ideas. One popular theory is called "cold dark matter (CDM)."

License information: CC BY 4.0
MPAA: G
Go to source: https://kids.frontiersin.org/article/10.3389/frym.2017.00029