You can detect "invisible" galaxies by their tidal effects on gas in galaxies: At the #aas217 this effect was validated in the case of M 51's obvious companion and others - then used to predict an unknown Milky Way companion at 80 AU with 1/100 the MW mass. See http://newscenter.berkeley.edu/2011/01/13/new-technique-could-pinpoint-galaxy-x for more.

You can detect "invisible" galaxies by their tidal effects on gas in galaxies: At the #aas217 this effect was validated in the case of M 51's obvious companion and others - then used to predict an unknown Milky Way companion at 80 AU with 1/100 the MW mass. See http://newscenter.berkeley.edu/2011/01/13/new-technique-could-pinpoint-galaxy-x for more.