(written from a Production point of view)
After the USS Enterprise-D experienced a series of events in which the ship momentarily phased out of normal space, Data measured the W particle inference. This confirmed one of his hypotheses, which was that the dark matter in the nebula was causing small gaps in the fabric of normal space. As the Enterprise passed through these gaps, a part of the ship briefly phased out.
The explanation given in the script was simplified for the episode, and the mention to the W particle was subsequently dropped. Nonetheless, there is an entry on W particle inference in the Star Trek Encyclopedia (3rd ed., p. 554).