Another way of prodding your thinking on this:
If you look in any basics physics book, you'll see some categorization
of all fundamental interactions like this:
- electromagnetic
- gravitational
- weak nuclear force
- strong nuclear force
So the obvious question is, where did these categorizations come from?
The ones that were known first were gravity and electromagnetism. When
nuclear interactions were studied, whatever gave people the idea that
there must be at least one more that is distinctly different than either
electromagnetism or gravity? A little diving even in oversimplified web
articles will give you a little insight, depending on your skill with a
search engine. (If you're not skilled with a search engine, then using
the web is pointless and a large waste of time, as you will not easily
find what you're looking for, and you'll come to the erroneous
conclusion that the answer is not there. If you're like this, then a
book is a much better approach.)