Why is there a gap in this image of supernova discoveries? I came across this gif showing supernova discoveries from the late 19th century to 2010. Here's the data in 2010:



Notice that there's a prominent region shaped like an inverted U in which there are few detection points, which I have crudely enclosed:



Why is this? Is it an artifact of detectors simple not observing that part of the sky, or is there some underlying astronomical reason?
The coordinate system in this image is RA and Dec. It is a coordinate system which uses the Earth's equator (projected onto the sky) as its midline.

The inverted U is the Milky Way. The Milky Way is full of dust and gas, and blocks our view of galaxies (and supernovae) behind it. There is enough dust in the plane of the galaxy to block our view in that direction. For example the galaxy IC 342 is one of the nearest galaxies, and would be brilliant if it were not close to the galactic plane. There may be other galaxies that are completely hidden.

Our galaxy's bulk not only hides supernovæ that are in other galaxies, it also hides most of the supernovæ that occur in the Milky Way