Look it up on Common Sense Media rather than asking the troglodytes on Reddit.

They're probably assuming an object, likely a black sphere, at a specific location or in a specific orbit. It's probably black because you have to specify emissivity and absorptivity numbers to calculate temperature in a radiation-dominated environment, and it's an easy shortcut to say both are 1.0 for a perfect black surface. Of course they may have a real surface property in mind like black anodized aluminum or whatever but then someone has to actually look up the properties. It's probably a sphere of area = 1 for simplicity.

Alternatively they may be referring to the random molecules and particles floating around in interstellar space. Near Earth there is atomic oxygen, each atom of which is relatively hot but there are so few of them that the overall environment isn't affected by their presence. Out in interstellar space there would be atoms that would be colder. That's just an assumption on my part, all of my experience is in LEO.

Please don't feed the trolls.