This beautiful galaxy has a high rate of star formation and a classical spiral structure, although one arm appears oddly to be stuck out.
This has been the subject of a lot of debate. Clearly this galaxy has interacted with another, but while the beauty is in what we can see, the science is often in what we cannot (at least with our eyes).
From the outstretched arm, a huge stream of hydrogen gas emanates into the void between the galaxies. Two theories explain this, one is that 250 million years ago a close encounter with another galaxy in the local group pulled at one of the arms, gas, dust and even stars were interchanged, but that has long since gone, and the arm is slowly returning to a more traditional look.
The second and much more radical theory is, that in the void between galaxies sits a dark galaxy, a galaxy made up of no stars and just dark matter, and that it was the interaction between this and M99 that has caused (and continues to pull on it) but is not visible to naked eye as it has no stars.
This is a point of debate between astronomers, particularly answering the question to why the dark matter galaxy only pulled at the hydrogen gas, and not the stars in the arm, as there would at least be some stars that would have found themselves pulled in, but there are none detectable.