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Look up 'quantum mechanics'
if you are looking for a more complex answer @henpen is probably right but i have not learned those things yet.
but if you want a kind of answer what i learned in Chemistry, that electrons that revolve around nucleus is a similar concept to Earth moving around the sun which is because of the gravitational field. likewise, the electrons have a charge in them and that charge is attracted to the charge in the nucleus. you know that positive charges and negative charges are attracted to each other.
i learned this 2 years ago so i did not use scientific language but i think u got the point.
the centrifugal force of electron spin balances the attraction force between the +ly charged nucleus(proton) and -ly charged electron.. that makes the electron to be in a definite orbit but not to fall fall on it!!