What is dark matter and dark energy?

"The first to suggest the existence of dark matter (using stellar velocities) was Dutch astronomer Jacobus Kapteyn in 1922.[18][19] Fellow Dutchman and radio astronomy pioneer Jan Oort also hypothesized the existence of dark matter in 1932.[19][20][21] Oort was studying stellar motions in the local galactic neighborhood and found that the mass in the galactic plane must be greater than what was observed, but this measurement was later determined to be erroneous.[22]"
SOURCE: en.wikipedia.org/wiki/Dark_matter

Jan Oort invented dark matter as a fudge factor to balance his calculations for the solar system. The idea was so well received that other astronomers proposed dark energy for a similar purpose. No such thing has ever been observed, but amateur scientists will shout insults at you if you point that out.

A big part of science is conjecture, which means making up "What if" scenarios. Often those scenarios get discussed a lot for a long time, and people begin to assume they are true just because they keep hearing them. Science is supposed to be based on observations, tests, and proofs. There is a strong tendency to accept conjectures, math models, and consensus of opinions instead. You need to be careful about accepting things that have not actually been observed.