You use the word 'believe' too often, to be believed.
Gravity is a theory and so is quantum mechanics, bonehead.
Scientific theory is based upon repeatable experiments and verifiable evidence. It's reliable and It's why you can fly in a plane, drive in a car and microwave your Hot Pockets.
And it's probably why you could undergo an MRI of your brain and confirm the atrophy of a never very robust critical thinking skills lobe.
Theory is not a lesser form of knowledge nor is it a guess.
A scientific theory is a well-substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment. Such fact-supported theories are not "guesses" but reliable accounts of the real world.