Not so long ago, in a galaxy not so far away, Earth-based astronomers discovered several clusters of stars with strange properties: the clusters' masses were much higher than their appearances let on.

And although we don't know exactly why these clusters have the masses that they do, scientists suggest that it has something to do with these clusters having a dark side, or rather they believe that the clusters, which contain less stars (making them darker than their brighter counterparts) contain black holes and even possibly some dark matter.

The clusters, located in the Milky Way's neighboring galaxy, Centaurus A, are globular star clusters. Although that sounds rather exotic, it's not: these huge spheres of thousands and thousands of stars are one of the most common types of systems in the Universe.

Astronomers study globular clusters because many remain from the beginnings of the Universe: most existed when the Universe was still a baby. And this new finding confirms something scientists suspected: not all star clusters are similar.

"Globular clusters and their constituent stars are keys to understanding the formation and evolution of galaxies," says Matt Taylor, a PhD student at the Pontificia Universidad Catolica de Chile. " For decades, astronomers thought that the stars that made up a given globular cluster all shared the same ages and chemical compositions - but we now know that they are stranger and more complicated creatures."

The Centaurus A galaxy probably has at least 2,000 globular star clusters. Most are brighter and more massive than any clusters in the Milky Way. And generally speaking, the brightness and higher mass should mean that they have more stars, but with some of the clusters in Centaurus A, that simply wasn't the case.

These dark clusters have fewer stars than their mass suggested, leaving astronomers scratching their heads. Do these clusters have black holes in their midst? Perhaps, but that still doesn't complete the picture. Do these clusters have dark matter? Scientists believe that such clusters don't contain dark matter, but it's possible that for some strange reason, these odd ones do.

At this point, astronomers are certain of one thing about these dark clusters: these clusters are different from anything we've observed yet. These findings suggest that such dark star clusters exist in other galaxies, perhaps in those far, far away. And this is yet one more clue about the mysteries of the Universe.

"We have stumbled on a new and mysterious class of star cluster!" says Taylor. "This shows that we still have much to learn about all aspects of globular cluster formation. It's an important result and we now need to find further examples of dark clusters around other galaxies."

[Photo Credit: ESO/Digitized Sky Survey]

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