Galaxy clusters are the largest gravitationally bound structures in the universe, keeping together hundreds of galaxies, dark matter, and intergalactic gas. Astronomers have now discovered that they also host the largest magnetic fields in the universe.

Such a discovery was possible by looking at the so-called radio relics, arc-like features of intergalactic plasma that emit a lot of radio waves. Relics are shaped by magnetic fields, and by studying its radio emission, astronomers can learn more about them.

According to a paper, published in Astronomy & Astrophysics, the magnetic fields are about the same strength as the Milky Way’s, but they appear to be more regular and less chaotic. Observations of four galaxy clusters have shown that the magnetic fields are as common as the gas between the galaxies in the cluster.

“We discovered the so far largest ordered magnetic fields in the universe, extending over 5-6 million light-years,” lead author Maja Kierdorf, from the Max Planck Institute for Radio Astronomy (MPIfR), said in a statement.

The astronomers established the strength of the magnetic fields, and while they are millions of times weaker than a common fridge magnet, they extend over regions 100 times the size of the Milky Way.  

The galaxy clusters that host these magnetic fields formed by the merging of smaller groups of galaxies. The team estimate that the clusters merged at velocities higher than 2,000 kilometers per second (4.5 million mph), which is greater than what was previously estimated using data from X-ray observatories.

Radio relics have been discovered in about 70 galaxy clusters so far, and these particular observations were conducted with the Effelsberg telescope, a 100-meter (330-foot) dish located in Germany.

“The Effelsberg radio telescope proved again to be an ideal instrument to detect magnetic fields in the universe,” added co-author Rainer Beck, also from MPIfR. “Now we can systematically search for ordered magnetic fields in galaxy clusters using polarized radio waves.”

It's possible that larger magnetic fields exist out there, some even bigger than the galaxy clusters that produce them.