Finding the need and the market

if there are approximately 7.700 million people in the world, why our main problem is finding a market?

There are many examples of big unicorns that were born only by listening to friends’ stories about a funny/unsatisfying situation. That clearly happens to a group of people, not only closed ones. The tricky part is to understand what is the need, and how many people suffer or have the same problem.

What do you do to find a market?