Originally Posted by
Xei
I'm not sure it is a mystery as to why space is expanding per se, but as to why the expansion is accelerating. I also didn't realise that galaxy formation was a mystery... as far as I understand it, the early universe was not homogeneous (due to quantum fluctuations), and so various volumes of hydrogen collapsed gravitationally. There may be some mystery about the details but as far as I know the general picture is understood. The double-slit experiment is kinda not a mystery either... as in, there's nothing inconsistent about it. Physicists largely just accept that's how reality works; there's only so far you can ask 'why' of something. The conceptual picture of abiogenesis I also wouldn't call mysterious, again in the sense that there isn't anything about it any more that seems completely absurd; we have quite a simple reductionist idea of how it would happen. There are very interesting unanswered questions though, such as whether any alternate biochemistries are possible, how likely each stage is, and how abundant the various stages of the life are elsewhere in the universe.
The main one I would add is intelligence (in the fundamental sense of the word). We still really have no idea how intelligence works, or how to approach the problem. Basic questions like 'what physically is a memory' or 'what physically is a thought' are unanswered. I would also add 'consciousness', in the sense of matter having qualia, but I'm sceptical about whether that's a question we could ever make any satisfactory progress on, even theoretically speaking.