The phrase "unseen mass" refers to something that cannot be seen or easily noticed, but still exists in a large quantity or has a significant impact. Full definition
This suggested there was some sort of unseen mass in a «halo» around the galaxy. (sciencemag.org)
The fact that the galaxies hold together suggests that unseen mass provides the gravity to hold them together. (discovermagazine.com)
A radical new model of gravity seems to account for bending of light by distant galaxies without invoking extra unseen mass whose identity remains mysterious (newscientist.com)