We have never seen dark matter and dark energy. Why do we think they exist?

Thanks to Euclid Satellite projected for the probe of dark matter/energy.

To maintain all the laws of physics constant,observational data must support therries.

Law I .
Einstein's GR : With directly proportional to Time starting from the big bang,Space and Mass(or Gravity) increases.

Law II.
Dark Matter + Dark Energy Density is constant for all the Time in a stable system of galaxies.