Regular matter fits that description too, but saying it doesn't interact with light is not the best way to say it. It's giving an assumption, that it's a black body because we cannot see it with our current tech. The flash light responder of my first comment is ironic, because you can think of a dark night with a flash light failing to illuminate things perpendicular to the beam. Space is vast, and we know physics does some strange things depending of the scale (tiny or huge). Have you pondered any of these questions: If there was enough light to illuminate everything in the universe, would space look black? When you think about solar systems, do you imagine stars at the center of them? Have you thought about solar systems that do not have the critical mass to ignite a star? What if the percentage of dark matter lines up with percentage of solar systems without a star to illuminate nearby objects? Is there really a large planet out in the Oort cloud, or is there that much little rocks out there we cannot see? And, on top of all that, I know for a fact there isn't solid evidence for any explanation yet, but people will not be scientific and disregard anything that's not the popular meme. But again, that is the common reaction through out history of new theories, until they are proven definitively. Even then people will believe the old norms in the face of facts. MIT is famous in teaching to brainstorm ever single possible solution, no matter how dumb or weird it is, because it might actually not be later on. Science isn't about sticking to a popular belief and defending it only, as that will stifle progress. There are lots of things we don't fully understand, and that's okay, but the unknown can be scary (and we have seen conservatives do some awful stuff in fear of changing their world views). We don't fully understand gravity as a "force". There are drugs we don't fully understand what exactly happens in our body, but see the side effects. We don't fully understand how the brain works. DNA was mapped but understanding it is going to take a very long time with research. I knew the type of responses I would get for not saying the popular theory (But hoped I would pleasantly surprised with mindfulness). And I do mean theory in the scientific definition, but most people treat it as scientific laws. Have you notice how people talk about the big bang theory, like they been there and seen it, the conviction in the way it's spoken, using that background radiation as the main source. I welcome new evidence, and will change my guesses/hypotheses when it comes, as all scientifically minded should. I just think it's wrong to clinch on to popular beliefs and not properly lay out assumptions.