I'd say it was the two galaxy sized bubbles immediately above and below the plane of the Milky Way that were just discovered but they used the Fermi Gamma-Ray Telescope for those.
http://www.telegraph.co.uk/science/space/8125127/Giant-space...
http://www.telegraph.co.uk/science/space/8125127/Giant-space...
Edit: Apparently, this: http://www.nasa.gov/home/hqnews/2010/nov/HQ_10-299_CHANDRA.h...
"Astronomers using NASA's Chandra X-ray Observatory have found evidence of the youngest black hole known to exist in our cosmic neighborhood"
"The 30-year-old object is a remnant of SN 1979C, a supernova in the galaxy M100 approximately 50 million light years from Earth."
Well, pretty underwhelming.