Hubble time! 🌌 A massive, spacetime-warping cluster of galaxies is the setting of this picture. The galaxy cluster in question is Abell 209, which is located 2.8 billion light-years away in the constellation Cetus (The Whale). This Hubble image of Abell 209 shows more than a hundred galaxies, but there’s more to this cluster than even Hubble’s discerning eye can see. Abell 209’s galaxies are separated by millions of light-years, and the seemingly empty space between the galaxies is actually filled with hot, diffuse gas that can be spotted only at X-ray wavelengths. An even more elusive occupant of this galaxy cluster is dark matter: a form of matter that does not interact with light. The Universe is understood to be comprised of 5% normal matter, 25% dark matter, and 70% dark energy Hubble observations like the ones used to create this image can help astronomers answer fundamental questions about our Universe, including mysteries surrounding dark matter and dark energy. These inves...