The name of this type of verse is indeed quite the oxymoron, as they have multiple Terminal Boxes. In fact, they're very dense. However, entities named them this way because every single object in them cannot be seen, and instead they blend into the black color of the rest of the "void".
You need special technology known as Seekrs to see any Terminal Boxes within a Pure "Void". The sheath, however, coating them is said to be very easy to traverse, making Seekrs essential to build a multi-Tbox civilization. Also, on a sillier note, trees grow on the sheaths which are known to bear fruits that taste of lemon and apples.
Pure "Voids" are normally contained in Loweffortverses due to being considered a LEV (low-effort verse), though they have a heat resistant variant able to be outside of a Loweffortverse and instead be directly contained by the next type of structure, Magma Realms. Civilizations inside the Loweffortverses consider that these variations are putting "a lot of effort in order to resist the heat" so they dont consider them as LEVs and, by consequence, as verses that need to be contained... dumb.