Sometimes the distortions are obvious, like in the Hubble image of a distant galaxy cluster above.

A galaxy cluster is a collection of galaxies that orbit one another bound together by gravity.

The textbook case is the Bullet Cluster, where one galaxy cluster is seen shooting through the other.

In the foreground of one new image is a galaxy cluster named Abell 2744, containing hundreds of galaxies.

Image: Light bends around the massive galaxy cluster Abell 2218 in this image from the Hubble Space Telescope.

At the center of the Perseus galaxy cluster is NGC 1275, a galaxy surrounded by mysteriously glowing gas filaments.

These observations told them how strong each galaxy cluster's gravitational pull was, from which its total mass could then be calculated.

According to Nasa, this ancient collection of galaxies presumably grew into a modern galaxy cluster similar to the massive ones seen today.

The increase in brightness of Icarus, the researchers say, is due to an additional magnification boost from a star within the galaxy cluster.

Captured by NASA's Chandra X-ray Observatory and combined with data from infrared and optical telescopes, this image shows the farthest galaxy cluster ever detected.

And if your birthday is on, say, 23 August, then your lucky image is the massive galaxy cluster Abell 2744.

This undated image from the Hubble Space Telescope shows a ghostly ring of dark matter in a galaxy cluster designated Cl 0024+17.

These plumes of gas contain as much mass as all the gas within 12,000 light-years of the center of the galaxy cluster M87 belongs to.

How do we measure gravity's pull in colliding galaxy clusters?

The large-scale structure of galaxies and galaxy clusters.

When viewed close up, the matter in the universe bunches up into stars, galaxies and galaxy clusters.