The 23rd dimension is home to an elusive and enigmatic race known to most cultures only as "The Shadow Dancers" or a variation upon that theme.
That is, cultures whose ships have returned from the 23rd dimension.
the 23rd dimension is composed of various planets, and a curious phenomenon, reversated pulsars. These pulsars absorb and refract light from its original color patterns, creating a space in which the sky looks black at night, because of the lack of light in the visible spectrums.
The Shadow Dancers use this to their full advantage, and their black wisp-like ships will stay in the shadows and dark corners, before alighting the blackness with sudden charges of plasmic energy, and then instantly warping and disappearing into the folds of blackened space.
The invaders were in for a fight. Or more accurately, and ambush.