“Dark matter” is a name that’s been given to a substance which many scientists believe may exist. If it exists, this substance could account for the amount of gravity that astronomers have observed in galaxies. Astronomers believe that more gravity is at work in galaxies than could be generated by the ordinary matter that they observe, that is, the number of stars and other heavenly bodies that they can see through telescopes or radio telescopes. Calculations indicate that there is “missing mass” in the galaxies. Estimates vary, but if it exists, dark matter could account for 27% of the matter in the universe.
One possibility is that the missing mass is present but invisible to us for some reason. If it’s present, it’s not believed to interact with light or other electromagnetic waves, like radio waves. That would account for why it can’t be seen through telescopes or radio telescopes. As they can’t see it, astronomers call what they believe may be hidden, “dark matter.”
Thus, the term “dark matter” is a placeholder. It’s a name that scientists have given to something which they can’t detect. If it does exist, it would explain phenomena regarding gravity that cannot currently be explained.
Dark matter and dark energy are different. While dark matter could account for “excess gravity” in the universe, “dark energy” is the name given to another unexplained aspect of the universe. Dark energy has been postulated as an explanation for the accelerating motion of galaxies away from each other.