The existence of dark matter was first proposed in the early 20^th century to explain why galaxies act like they’re actually heavier than they appear to be. Today, we have yet to see it because, by definition, dark matter does not interact with electromagnetic radiation (which includes light and everything from radio waves to gamma rays). The stuff that composes 27% of the mass and energy of the observable universe is basically invisible.

Physicists already agree that no substance we currently know of could make up dark matter. The most common theory is that dark matter particles are quite heavy, 100 times the weight of a proton and nearly dense enough to cause black holes. Despite countless investigations, there has been no conclusive evidence of these super-heavy particles.

New theories put to the test

But some researchers, including those at Stockholm University, have another idea.

They think that the dark matter particles could actually be really, really light. These super-light axion-like particles (ALPs) would also really difficult detect because they wouldn’t break apart when they bang together and would survive longer than the universe itself.

The Fermi Gamma-ray Space Telescope was launched by NASA in 2008 specifically to test theories of matter and phenomena at the edge of our understanding of the universe. Investigators include more than 400 scientists in 12 countries, with the United States, France, Italy, Japan and Sweden talking the lead. The data, along with tools to sift through them, are available to anyone on the NASA website.

To test the ALP (aka super-light dark matter) theory, the team at Stockholm University, looked at the central galaxy of the Perseus cluster, which features a super-massive, spinning black hole with a plasma stream that puts out gamma radiation. The thinking is that ALPs could very well turn into light in the magnetic field in the hot gas that fills the cluster, and that this would cause detectable distortions.

Dark matter not-not super-light

“We did not find any such features above the level of statistical fluctuations which enabled us to rule out certain ALP models that would contribute up to 4% to the overall dark-matter content,” said Manuel. To translate, the Oskar Klein team found nothing to show that dark matter couldn’t be made out of ALPs. "While we don't yet know what dark matter is, our results show we can probe axion-like models and provide the strongest constraints to date for certain masses," Meyer said. “Remarkably, we reached a sensitivity we thought would only be possible in a dedicated laboratory experiment, which is quite a testament to Fermi.”

One small step for humanity, one small step toward a better understanding of our universe.

More information at Oskar Klein Centre website

Text: Kimberly Parke