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

The Euclid telescope will trace the signatures of dark energy in the distribution of galaxies across the universe,

Scientists think that only about 5% of the stuff that makes up the universe is known to us. 

Yes, you are reading that right. Cosmological models show that invisible energy and matter must make up a combined 95% of the universe for it to work according to existing theories.