The Subspace Bomb is the primary weapon of the Subspace Army. When detonated, it removes the surrounding area from the world and deposits it into Subspace, leaving a dark void behind. The effect is additive; a larger area can be affected by causing multiple bombs to go off at once. Each bomb requires two Robotic Operating Buddies to function (with the exception of Galleom, who has a self-sufficient Subspace Bomb in his head). The bomb keeps hold of the ROB's arms, so two ROBs have to be sacrificed for each bomb. Why exactly the bombs were designed to act this way is unknown, although it is possible that they require the ROBs' electric power (this would also explain why Galleom's does not, as his robotic body could provide sufficient power).
Subspace Bombs were designed and built on the Isle of Ancients. As such, there is a large factory on the island, able to store dozens of bombs at once.
The average Subspace Bomb has a detonation timer of three minutes. Presumably, this allows Subspace Army members to leave the scene before the explosion. Galleom has a specialized subspace bomb attached slightly near the back of his neck. Galleom's version, however, is set for only fifteen seconds. Since Galleom likely tried to blow up the bomb as soon as possible, it's likely it needed fifteen seconds to charge.
Since Subspace Bombs have always succeeded in exploding (unless they are damaged before activation), it's unclear whether the countdown can be stopped. It's obvious that the Smashers think so, as they've tried repeatedly (but failed) to remove the ROBs from the sockets or otherwise disable them.
Trophy Info
Subspace Bomb Trophy is one of the many trophies gained on Super Smash Bros. Brawl. The trophy is under the Subspace Emissary category if one searches by game title or under enemies if one searches otherwise. It is unlocked by completing Boss Battles on easy difficulty.
A weapon that, when it explodes, draws every part of this world within its blast range into Subspace. The areas drawn into Subspace this way float about in bubbles as individual colonies. To detonate a bomb, two R.O.B.s must be destroyed in the process. The Ancient Minister is bitterly aggrieved by this fact.