Now there's an interesting question. Its the sort of simple question that probably occurs to most people at one time or another and then ...
Maybe you've thought about it a bit, and asked some other people what they think, and that helped you think about it ... but also made it seem a more difficult problem than you first thought it was? Maybe you've even done some reading, and that helped too ... but also made it seem an even more difficult problem? And so you've sort of filed it under "interesting but ..." and decided ... that's not an answerable question.
Maybe "what is 'beauty?' isn't in fact an answerable question, but maybe we could break it down into some more answerable questions, and perhaps that way we could at least understand the original question better? Here are a couple that come to mind:
One might think one "knows" the answer to questions like those above, or at least that someone else does, but it often turns out that actually making some observations oneself helps one to see the question differently. And making them in a way that other people can see them too helps in comparing how one thinks about things with how others think about them. So, let's take a crack at it. Let's do some "research" together and see what we learn from it.