Transphasic torpedo

Only seen in the Voyager episode "Endgame", the Transphasic Torpedo is apparently able to pass through enemy shields and perhaps even phase through the hull of enemy ships. These weapons remain an almost complete mystery, except for the obvious fact that they do a very good job of destroying Borg Cubes. These cubes can be relatively easily destroyed through attacks to 'weak points', so this is the most conservative explanation for their effects. Of course, a torpedo with the ability to 'phase' through shielding and then strike in the center of - say - a reactor would likely cause serious damage to most ships.

The absence of Transphasic Torpedo technology in other starships or during other conflicts, however, must be explained. Several possibilties exist:

1. Voyager was unable to transmit the data back to the Alpha Quadrant, so Starfleet HQ never put Transphasic Torpedoes into full fledged construction.
2. As a slight variation on #1, the Transphasic Torpedoes were invented in a alternate future timeline, so the information would not have been available to starfleet HQ in the regular timeline.
3. The materials needed to produce a Transphasic Torpedo are rare/expensive/not readily available in the Alpha Quadrant.
4. The Transphasic Torpedoes were confiscated by Starfleet Temporal Investigations for whatever reason.

Some weaker explanations are:

1. Transphasic Torpedoes only were able to pass through Borg shields because the Borg's shields function based on phase-adaptation, and thus Transphasic Torpedoes go right through them. (Note: this does not explain the hull-penetration).
2. Transphasic Torpedoes are relatively unstable and cannot be stored for long periods of time, as they are likely to 'phase' out of existence. (No on-screen reference to Transphasic Torpedo instability)