To What Extent are Molecular Clouds Geographic Barriers in Space?
In a sci-fi universe I've been working on for quite a while, I have different civilizations that are initially unaware of each other's existence, having forgotten their recent common origins. The setting is on the harder side of sci-fi, but it would add much to the political and cartographic "landscape" of the setting if I could say that molecular clouds add some defining features to the void in leu of oceans and mountain ranges. To be clear, I am not suggesting adding insane amounts of mass into space to create literal impassable barriers; that would just have collapsed into very massive stars (since thats how stars are formed. Molecular clouds are also sometimes called star-forming regions). The material is not always dense enough to collapse on its own, however, and so we have these large regions of space that appear opaque. The idea would be to have these features block the radio signals from the different civilizations initially, and make space travel through such regions risky or at least rough on the hull (relativistic spaceflight is a risky proposition in the best conditions), which would be why these separate civilizations remained so for so long. But I don't know enough about these gargantuan structures to really definitively say that this is the case. One thing I do know is that these regions of space are extremely dense, with hundreds of atoms every cubic centimeter compared to only about 1 atom every 10 cubic centimeters, mostly hydrogen, in the interstellar medium in general. Additionally, molecular clouds consist of dust and, deeper within, frozen hydrogen particulates and some of the coldest temperatures in the universe, at least in the dark nebula. Some of these regions may have their own magnetic fields deep within.
My question, then, is how the presence of molecular clouds would impact communication and travel between interstellar civilizations, assuming plausible technologies and millions of inhabited star systems as the stage. Specifically, what I'm looking to learn is how these factors should reasonably affect travel though these regions using technology as we understand it, and more importantly, how communications with radiation as we understand it would be affected, knowing the physics and properties of molecular clouds. It could be that travel and communication would not be affected at all, or it could be that such high-density regions prove to be near-impassable barriers. Either way, it would be good to know, or to at least hear some educated opinions on the matter.
Image source: https://astronomy.swin.edu.au/cosmos/d/Dark+Nebula