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

We have never seen dark matter and dark energy, so why do we think they exist? And what would happen if Europe's new space telescope Euclid finds evidence that they don't exist after all?

We have never seen dark matter and dark energy. Why do we think they exist?
We have never seen dark matter and dark energy, so why do we think they exist? And what would happen if Europe's new space telescope Euclid finds evidence that they don't exist after all?