John Timmer points out the problem with a press release:
Scientists who are attempting to communicate with the public often use metaphors to help people understand something that's otherwise hard to comprehend. Sadly, some of them wind up obscuring more than clarifying. I think one such example was the discovery of a giant ball of gas discovered amidst a galaxy cluster. Many stories used similar language of terming this "comet-like," undoubtedly taking the language from the press release. It's 5 million light years long, moving at 750 kilometers per second, it is moving between entire galaxies, and is estimated to be at a temperature of 46 million degrees Celsius. Other than that, just like a comet.