Other than Pauli, you mean? It was hypothesized around 1930. Discovered in 1934.

The basis of scientific method is following:

1. You notice something.

2. You ask a question.

3. Form a hypothesis.

4. Experiment.

5. Analyze.

6. Draw conclusions.

After linear momentum was missing in collisions!

Pauli speculated that neutrinos would exist after an experimental anomaly.

No one really saw it coming.

No one saw 3 generations coming either, for example.

In fact, people were baffled.

Yes, but what is FCC supposed to look for?

If it's just Dark Matter, and Matter/Anti-matter asymmetry, what theoretical framework is it going to explain it? Will it explain it, or will it just do "Your asymmetric partners are in another Order of Magnitude collider"? Or maybe there are actually 34 dimensions and not like four.