Doom metal is a form of heavy metal music that typically employs very slow tempos, low-tuned guitars and a much 'thicker' or 'heavier' sound than other metal genres. Both the music and the lyrics intend to evoke a sense of despair, dread, and impending doom. Quote from the ubiquitous Wikipedia...
And yes they did sing about impending doom especially on The Colourblind album and Microscopic View of a Telescopic World.