Answer:
!dk if this answers your question.
Step-by-step explanation:
Astronomers have spent a lot of time measuring the rotation rates of galaxies. The rates do not fall off with distance as they would if visible matter was all there was. So, unlike the solar system, where the rate falls with distance, in galaxies the rate at the edge is too high to hold together, unless there is more mass than can be seen. This was the first indication of what is now termed 'dark matter'. It exists as huge blobs of mass that envelope galaxies. What it is made of, no one knows yet.