Antimatter mine
From Memory Alpha, the free Star Trek reference
An antimatter mine is a relatively small device the size of a small probe that contains a large amount of antimatter. On board sensors detect close-range starships and the mine moves towards them. On impact, the containment fields within the mine collapse, dispersing antimatter onto the ship's hull causing a large number of instant annihilations, resulting in an explosion.
In 2369 the crew of the USS Enterprise-D used 500 antimatter mines of varied sizes by order of Captain Edward Jellico to force a Cardassian fleet hidden into the McAllister C-5 Nebula to withdraw. The mines were placed surreptitiously by a shuttle and tacked themselves on the hull of the Cardassian ships. (TNG: "Chain of Command, Part II")
The object pictured is actually a US Naval Sonobuoy shipping container. These heavy plastic containers are generally disposed of once the expendable contents are unpacked for use. The TV and film industry has used these cheap and plentiful containers for years in many science fiction productions. They often are used as special plot devices, interior set components, and most often as generic cargo containers.
