While some scientists look to define dark matter from theoretical physics, others are out there trying to understand it from how it moves luminous matter. 

Dark matter is pretty much everywhere in our universe, but it has its highest densities in galaxy clusters. This has been determined by looking at how the gravitational pulls of dark matter twist the light of background objects. While this technique, called weak lensing, can get us good information on the distribution of dark matter, it doesn’t give us a very detailed image. 

Coming to the rescue is intracluster light: light from stars that have been torn from galaxies and are now living in the spaces between galaxies. This light is extremely faint and requires near Herculean observing efforts to see. The Dark Energy Survey, with its sensitive camera and long observations, has taken on this task and found gold or at least intracluster light. Researcher Yuanyuan Zhang explains: Observationally we discovered that intracluster light is a pretty good radial tracer of dark matter. That means that where intracluster light is relatively bright, the dark matter is relatively dense.

These results were checked against weak gravitational lensing results, and everything matched.

This work combines data from 500 different clusters within 3.3 billion light-years and helps us understand how dark matter is distributed in our modern universe. Seeing intracluster light in more distant galaxies is a goal for this team. Since the light from distant clusters has been traveling for longer, we can essentially look back in time by looking at farther away systems. 

In this case, distant clusters could allow us to understand how the distribution of dark matter has changed over time. Zhang goes on to say: My dream is to go all the way to … 10 billion light-years. Studies say that’s when the ICL has just started to evolve.” 

Getting the dreamed-of data won’t be easy, but we don’t do science because it’s easy. We wish Zhang and her team clear skies.

More Information

Fermilab press release

“Is diffuse intracluster light a good tracer of the galaxy cluster matter distribution?“, H Sampaio-Santos et al., 2020 November 26, Monthly Notices of the Royal Astronomical Society