Here are some “pictures” of dark matter. You can detect it by its effect on light coming from sources behind it, through gravitational lensing.
IIRC, it was hypothesized because the mass of visible galaxies was not large enough to account for the lensing observed.
Inflation made predictions about the lumpiness of the cosmic background radiation - predictions which were confirmed.
Dark energy is required to explain the observed expansion of the universe, but I’m not sure it has been observed as such, unlike the other two. I’m not sure it has made it to theory status yet, but I could be wrong.
Now could the OP explain why he thinks these things are “beliefs” aside from not knowing much about cosmology? There are plenty of books and popular articles on this, including a full book on gravitational lensing.