In, say 1975, there was only one kind of 'stuff' in the Universe, matter made of protons and neutrons. By 1985 there was a consensus that this was not enough, the gravity of the stars in galaxies was not sufficient to hold them together. Something else was there, vast, mysterious and imperceptible, only observable by its effects on its surroundings. Imagine HG Wells 'Invisible Man' moving unseen through a room until he knocks over a vase. There was much more of this dark matter out there than stuff we could experience directly.
By 2005 it was clear that there was more going on out there. Another unseen and unknown something was pushing the galaxies apart. This is dark energy, a force which permeates all space. Astonishingly weak, dark energy can still move galaxies given aeons of time.
No one knows what dark energy and dark matter are; researchers study the light of far off galaxies to see how these elusive quantities effect them. The image shows the galaxy cluster Abell 1689, light from distant galaxies is bent by the gravity from galaxies closer to us but on the same line of sight creating a text-book example of a gravitational lens. This gravity is mainly, we now know, exerted by the dark matter in the galaxies (the blue fuzz in the image is not real, it is a superimposed map of matter distribution). At the same time, however, dark energy is pushing the galaxies apart adding a subtle additional distortion to this ancient light.
Dark matter and dark energy are utter mysteries today but so were electricity, magnetism and radioactive decay once. In some future time we will understand dark energy and matter just as well. Dare I speculate that one day we shall apply them? (I'm holding out for spindizzies myself.)
Image credit: NASA, ESA, E. Jullo (JPL/LAM), P. Natarajan (Yale) and J-P. Kneib (LAM).