It’s got to be Dark Matter. So many astrophysicists have spent so much time thinking about this stuff and all they’re really sure about is that there must be much more matter in the Universe than we can see, and yet we never actually seem any closer to knowing what it is. In conjunction with Dark Energy it just leaves the layman with the awkward possibility that maybe our model of The Universe is just fundamentally flawed somehow.
Always felt like it was kind of rude that Susan Sarandon kept this guy’s name long after they were married, and got way more famous with it. Worth it for the alliteration.
Whenever you visit a Roman Fort marked on an O.S. map it’s pretty much always just a barely noticeable hump in the field where there may once have been a wall - if you’re lucky enough to see anything.