Find the direction of the net electric force exerted on a charge q, located at the center of the square, for the following arrangement of charge: the two positive charges are on the top corners, and the two negative charges are on the bottom corners.

Downwards towards the square bottom

Explanation:

The direction of the net electric force exerted on the charge +Q is downwards, towards the bottom of the square as the two positive charges at the top are repulsive on the charge Q pushing it down while the negative charges on the bottom corners tend to attract the positive charge Q downwards pulling it towards them

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.

3. dark matter

empty space could also be a simple vacuum