Let me put it this way: in all of the industries I've been exposed to, I've never encountered one that had a) no pain points or b) companies entirely happy with their current vendors.

Note that you don't necessarily need to find a market that's underserved. Plenty of markets are served, but not well. Sometimes, doing something marginally better and/or doing it for less than the entrenched competition is all you need to create opportunity.

The size of that opportunity will vary of course. A lot of wantrapreneurs look at huge markets and think in terms of 8, 9 and 10 figures, but if you're more realistic (i.e. you'd be happy with a six or seven-figure annual top line with healthy margins), I would suggest that opportunity is abundant. The easiest way to find that opportunity? If you meet people in unsexy markets, talk to them. Most wantrapreneurs won't give these folks the time of day.