There are a bunch of good reasons to believe there really is matter there that we can't detect aside from its gravity. Here's a talk: <https://www.youtube.com/

There are a bunch of good reasons to believe there really is matter there that we can't detect aside from its gravity. Here's a talk: <https://www.youtube.com/watch?v=h8KV9Ht5o_M https://www.youtube.com/watch?v=h8KV9Ht5o_M>.