We only see 5% of the universe

Astrophysicist Katie Mack (astrokatie.com) created this startling pie-chart to illustrate the ratio of observed matter to dark matter and energy, the invisible bulk of the universe.

From her 2014 article, The Dark Matter Poltergeist:

So what do we know? We know dark matter is real—the evidence is overwhelming. Something must be responsible for the extra gravity messing with the motions of stars and galaxies. If that doesn't convince you, you can look to gravitational lensing—the bending of light around massive objects. The presence of dark matter accounts for the way the light from distant stars and galaxies is distorted as it travels through the universe, following the gravitationally induced curving of space-time itself. If motion and lensing don't convince you, look at the evidence that galaxies existed within a billion years of the Big Bang. Without dark matter as a kind of cosmic glue, galaxies would have taken much longer to form, as the gravity of gas and dust and stars had to fight against the pressure of all that matter colliding and heating up. Or just take a look at the Bullet Cluster. It's the aftermath of a cosmic collision in which clusters of galaxies collided but the bulk of the matter passed right through the collision, in the way only ghostly dark matter could.