The galaxy has a normal looking arm and an extended arm that is less tightly wound. A bridge of neutral hydrogen gas links NGC 4254 with VIRGOHI21. The gravity from the dark galaxy VIRGOHI21 appears to have distorted M99 and drawn out the gas bridge, as the two galaxy-sized objects have a close encounter, before they go their separate ways. It is expected that the drawn out arm will relax to match the normal arm once the encounter is over. Three supernovae have been observed in this galaxy.
Messier 99 was discovered by Pierre M?chain on March 17, 1781 along . The discovery was then reported to Charles Messier, who included the object in the Messier Catalogue, which was the first astronomical catalogue of star clusters, nebulae, and galaxies.