Hoag's object
Many of the details of the galaxy remain a mystery, especially how it formed. "Classic" ring galaxies are usually formed by the collision of a small galaxy with a larger disk-shaped galaxy.  This collision produces a density wave in the disk which leads to a characteristic ring-like appearance. Such an event, 2–3 billion years in the past, may have formed polar-ring galaxies. However, there is no sign of any second galaxy which would have acted as the "bullet", and the core of Hoag's object has a very low velocity relative to the ring, making the typical formation hypothesis unlikely.