In the 1970s, scientists noticed something strange about the motion of galaxies. All the matter at the edges of spiral galaxies was rotating just as fast as material in the inner parts of the galaxies. But according to the laws of gravity, objects on the outskirts should be moving slower.

The explanation: A form of matter called dark matter that does not directly interact with light.

Many scientists now believe that more than 80 percent of the matter of the universe is locked away in mysterious, as yet undetected, particles of dark matter, which affect everything from how objects move within a galaxy, to how galaxies and galaxy clusters clump together in the first place.

This dark matter extends far beyond the reaches of the farthest stars in the galaxy, forming what scientists call a dark matter halo. While stars within the galaxy all rotate in a neat, organized disk, these dark matter particles are like a swarm of bees, moving chaotically in random directions, which keeps them puffed up to balance the inward pull of gravity.

Bhuvnesh Jain, a physics professor in Penn’s School of Arts & Sciences, and postdoctoral student Eric Baxter are conducting research that could give new insights into the structure of these halos. The researchers wanted to investigate whether these dark matter halos have an edge or boundary.

Using a galaxy survey called the Sloan Digital Sky Survey, Baxter and Jain looked at the distribution of galaxies around clusters. They formed a team of experts at the University of Chicago and other institutions around the world to examine thousands of galaxy clusters. Employing statistical tools to do a joint analysis of several million galaxies around them, they found a drop at the edge of the cluster.

In addition to seeing this edge, when they looked at galaxy distribution, the researchers also saw evidence of it in the form of galaxy colors.

When a galaxy is full of gas and forming many big, hot stars, the heat causes it to appear blue when scientists take images of it.

“But those big stars live very short lives,” Baxter says. “They blow up. What you’re left with are these smaller, older stars that live for long periods of time, and those are red.”

Scientists expect that galaxies that have spent more time orbiting through a cluster will appear red, while galaxies that are just starting to fall in will appear blue.

The researchers noticed a sudden shift in the colors of galaxies right at the boundary, providing them with more evidence that dark matter halos have an edge.

“It was really interesting and surprising to see this sharp change in colors,” Jain says, “because the change of galaxy colors is a very slow and complex process.”

The researchers are working on another paper using a deeper survey of more than 100 million galaxies called the Dark Energy Survey.

Instead of measuring the distribution of galaxies, the researchers are using an astrophysical phenomenon called gravitational lensing to probe the dark matter halos. In gravitational lensing, light coming to an observer bends as matter exerts gravitational force on it.

The researchers can analyze images of the sky to see how clusters stretch images of the galaxies behind them.

The researchers hope that their research will contribute to a better understanding of the mysterious substance that makes up about 80 percent of matter in the universe. If they can mark the edge of a dark matter halo, it would allow them to test things like Einstein’s theory of gravity and the nature of dark matter.

“It’s just a new way of looking at clusters,” Jain says. “Once you find the boundary, you can study both the standard physics of how galaxies interact with the cluster and the possible unknown physics of what the nature of dark matter and gravity is.”