In recent years, a number of unexplained phenomena in Messier 99, one of over a thousand galaxies that make up the Virgo Cluster, the closest cluster of galaxies to us, around 50 million light-years away, have been studied by astronomers. Messier 99 is a so-called grand design spiral (image below), with long, large and clearly defined spiral arms — giving it a structure somewhat similar to the Milky Way, although it may have been distorted by gravity from the possible dark galaxy VIRGOHI21.