Wasn't it mentioned that dilithium crystals are able to control/direct/channel anti-matter reactions, hence why they are used?
This was a backstage rationalization that was probably first sketched out for ST:TMP, then used as an assumption in TNG, but first got an onscreen mention in the final season of ENT only...
However, dilithium is not how they
contain antimatter. That bit is explicated in TNG "Contagion" with its technobabble about "magnetic seals", an active system that can at least theoretically be turned off (to disastrous effect), rather than a passive material.
The whole chapter and verse about antimatter containment in the TNG Tech Manual is never quoted on screen, but onscreen Trek made after the writing of that manual conforms to what is written there, and thus tends to support the idea that forcefields (possibly called "magnetic fields" even if the truth is more complex) keep annihilation at bay. And forcefields do exhibit a glow when at work.
How torpedoes contain their antimatter (the presence of which only became canon fact in late TNG) is never made completely clear, but DS9 "Tribunal" shows that the warheads achieving this are a highly sought-after commodity, and something your basic food replicator can never hope to manufacture (or else the Maquis would have had no reason to steal the things). Yet the food replicator
can manufacture things as complex as working phasers ("Civil Defense"), probability-altering alien devices ("Rivals"), and living tissue ("Emanations"), not to mention food that tastes all right. So the mojo of the torpedo warhead is nothing to be sneered at!
Realistically, considering the speeds these things are fired at (can be fired at FTL), we shouldn't really be seeing anything except a minor flash at a torpedo tube and another flash when a torpedo hits its target less than a second later
But we shouldn't be seeing starships at warp, either. So it might be a good idea to assume that space combat is shown in extreme slow motion, via a camera capable of capturing FTL phenomena.
Timo Saloniemi