A dark matter nebula whose distortions caused objects and people aboard the Enterprise to phase in and out of normal space.
The Mar Oscura Nebula is a dark matter nebula that the Enterprise enters for study. Pockets of dark matter within it create distortions that cause objects aboard the ship to shift position or vanish.
In In Theory, the effects grow dangerous when a crew member is caught inside a deck as it phases. Picard pilots a shuttlecraft ahead of the ship to find a safe route out, while the main story follows Data's experimental romance with Jenna D'Sora.