Rydberg atoms are atoms orbitted by electrons in an excited state which leaves them barely attached to their nucleii, often for surprisingly long time periods. Rydberg Matter is basically clusters of Rydberg atoms attracted to each other and forming stable small n clusters. These small clusters typically form “atomic snowflakes”, but these in turn link up in long chains. Collected together in space Rydberg matter composed of interstellar hydrogen is largely transparent to radio, visible and IR light, thus it’s almost impossible to see… and possibly a billion times denser than the 106 protons per m3 that makes up most of the ISM. That’s bad news for interstellar travel as running into the stuff at high-speed (~0.1 c or more) would cause incredible meteoric levels of heating ~25,000 K.
An explanation for Fermi’s Paradox? Unsure. The chief researcher on Rydberg Matter seems kind of solitary in his research, perhaps indicating it’s dubious nature. More data from other researchers would be helpful.