Ask HN: What are some examples of small but rapidly growing markets?

I watched Sam Altman's amazing "How to Start a Startup" talk yesterday. Sam mentions that it's usually advantageous to build something for a market that is small and rapidly growing. One reason being that such markets aren't yet saturated with solutions and there's still stuff "obviously missing" - unlike in big, established markets. So, question is - What are some examples of small but rapidly growing markets? I have thought of a few, but am just interested in other perspectives.