Hubble celebrates spooky season with Abell 611, a spider’s web held together by a dark secret

In celebration of Halloween, Hubble brings you this inky image of the galaxy cluster Abell 611, located more than 1000 megaparsecs, or about 3.2 billion light-years, from Earth. Like all galaxy clusters, Abell 611’s continued existence creates a mystery for astronomers.

In particular, there does not appear to be enough mass contained within the web of its rapidly rotating component galaxies to prevent the cluster from flying apart. This is a well-established issue in astronomy with very massive structures like galaxies and galaxy clusters—they just don’t seem to contain enough combined mass to stay whole.

Interestingly, this problem does not appear on smaller cosmic scales. For example, the transit of the planets of the solar system around the sun can be calculated relatively easily using the masses and positions of the planets and the sun. No extra mass is needed to explain the integrity of the solar system or other star-planet systems. So why does this intuitive rule break down on a larger scale?

The prevailing theory is that the universe contains vast amounts of a substance known as dark matter. While the name may sound ominous, “dark” simply refers to the fact that this unknown quantity doesn’t seem to interact with light like other matter—neither emitting, reflecting, or absorbing any part of the electromagnetic spectrum. This dark quality makes dark matter incredibly difficult to characterize, although various possibilities have been hypothesized.

Essentially, most candidates for dark matter fall into one of two categories: some kind of particle that exists in massive amounts throughout the universe, but for some reason doesn’t interact with light like other particles. or some type of massive object also exists in great abundance throughout the universe, but does not lend itself to detection using current telescope technology.

Two of the more peculiar dark matter candidates fall into the first and second categories respectively. Weakly interacting massive particles (WIMPs) are hypothetical subatomic particles that do not interact with photons—in other words, they do not interact with light. Massive astrophysical compact halo objects (MACHOs) are a hypothetical set of very massive objects made (as opposed to WIMPs) of a type of matter we already know, but which are extremely difficult to observe as they emit so little light.

Despite enormous effort, however, no definitive evidence has been found for WIMPs, MACHOs, or any other form of dark matter.

If dark matter remains stubbornly undetectable, fortunately it is easily quantifiable. In fact, galaxy clusters like Abell 611 are ideal laboratories for quantifying dark matter, because of the abundant evidence of gravitational lensing visible within the cluster. An example of lensing is perhaps most clearly visible in the center of the image, to the left of the cluster’s bright core, where a light curve can be seen. This curve is light from a more distant source that has been bent and distorted (or “lensed”) by Abell 611’s massive mass.

The degree to which light has been bent by the cluster can be used to measure its true mass. This can then be compared to an estimate of its mass derived from all visible components of the cluster. The difference between the calculated mass and the observed mass is amazing. In fact, zooming in, astronomers currently estimate that about 85% of the matter in the universe is dark matter.

Even if the mystery of what holds together the cosmic web of galaxies inside Abell 611 remains unsolved, we can still enjoy this image and the fascinating science—both well-established and theorized—that takes place within it.

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