Was a scientific discovery, like fundamental particles attracting to each other, or was it just a general observation that things of different nature tend to draw together?
Edit:
Damn that extra "o" in the title.
What you seem to be talking about is Laws of Attraction. This dates as far back as Empedocles’ (490 – 430 BC) and his conception of philia (attractive force) and neikos (repulsive force). It's known as the "Law of Affinity," where you can read about it in just about any physics book as it's a foundation of modern physics such as Newton's laws on planetary motion.
When you're referring to "opposites attract," I assume you're referring to relationships/marriage in general. This theory can be attributed to a sociologist named Robert F. Winch that took the theory of attraction in a different direction and suggested that opposites attract. The paper was published in 1958.
Could you be a bit more specific? Do you mean opposites attract in a scientific nature, or how one would talk about a couple having different personality traits and saying "opposites attract!" I'm sure you could make a case that this observation has been made since males and females were attracted to each other and humans had the intellectual capability of pointing out that males and females are in fact not the same. The yin yang has been around for thousands of years, is that more in line with what you mean?