The large gray box on the bottom left is the Antimatter Fuel Container, which feeds into the bottom end of the Vertical Intermix Chamber
I personally don't think that the Antimatter Fuel Container would be that large (Assuming it's the big grey thing, if it's the circular thing, different story)
1 gram of antimatter is about 3 hiroshima bombs worth of explosive energy.
Another thing to point out is that antimatter must be kept from touching any matter, this would mean keeping it in a vacuum, magnetically kept away from the walls. If it were to touch a wall of it's container, the resulting explosion would be horrific.
I don't know what the star trek version of antimatter would do, but this is real life antimatter I'm talking about.