Some people believe that a neutrino soup of sorts prevails even in the emptiest space although I don't see how anybody could hope to prove that.
Way upwards of 99% of the mass of the universe is in plasma form and the main thing which aggregates plasma into solid things like galaxies and stars and planets is the Z-Pinch effect which is associated with the cosmic Birkeland currents which move through plasmas. That is why you find strings of galaxies and why, as Peratt and Lerner note in the video, solid material in the universe is "clumpy" beyond what could be explained by "big bang". Certainly the big bang offers no way to picture strings of galaxies forming.
There is no nothingness between the galaxies. If I remember correctly, the space generally has about 1 molecule per cubic centimeter of matter.
There is also a lot of stuff that is not visible at normal wavelengths. I think Uranametria 2000 startcharts have incorporated radio telescope findings with the visible light results from the Hubble telescope. Vast fields of dust and electrons. IIRC, the last edition is the one with the radio objects.
It is really facscinating. Lots of stuff out there.