In the five years since Netflix started streaming original series like the Emmy-winning “House of Cards” and “Master of None,” the shows have had a question hanging over them: How many people are watching?

Outside of Netflix, nobody knows the answer.

That’s because, much to the frustration of those in the industry who would like to have a firm idea of just how popular those shows are, the streaming services — Amazon and Hulu included — have been fiercely protective of their numbers.

Now, Nielsen, the 94-year-old company that for decades has had an effective monopoly on measuring television ratings in the United States, has announced that it has found a way into the great unknown of Netflix viewership.

What Nielsen’s data shows, exactly, and how rigorously it is measured remains something of a mystery, because Nielsen did not release the data publicly. The move, however, is a step toward finding a reliable third-party ratings system for streaming services.