For a cool outer space depth charge explosion, use a screwdriver and run it down the low E string of a guitar. EQ the sound so the high end is more prominent at the start of the overall sound, then gradually (about 2 seconds in) EQ the low end to be more prominent and the high ends fade off.
Then take a recording of an explosion (made earlier by hitting a cardboard box) – no debris is best -, increase the low end of the sound to the extent you really only get a large boom with very little actual crack. Add some reverb and also a little delay to the guitar sound only and you end up with a very massive, over-the-top depth charge sound.
– Thomas Harris