A graphic featuring Quark population, Kei and Yuri (bottom right)
A quark is a type of subatomic particle which was displayed on a starship's sensor readings of subspace particle emissions.
Quarks were among the particles listed in the computer's sub-space emission scan read out on the bridge station viewscreens of USS Voyager. (Star Trek: Voyager video playback set artwork)
The "sub-space emission scan 247" screen was a computer graphic artwork played continuously on some monitors on the set of Star Trek: Voyager, aboard Voyager and probably many other starships of the same era. The screen is reproduced legibly in Stephen Edward Poe's A Vision of the Future - Star Trek: Voyager companion book.
The script of Star Trek contained some additional questions and answers that could be used in the skill dome scene, including one that requested the electrical charge of an up quark.