Asolenoid on its side has a current running through it. the needle of a compass held upright in front of the solenoid points to the left. what happens to the direction of the compass needle if the direction of the current is reversed?

direction of magnetic field will change as the direction of current will be changed. So here if we reverse the direction of current then direction of field also reversed.

Explanation:

as we know by the formula of solenoid that magnetic field along its axis is given as

here we know that

N = number of turns

i = current

L = length of solenoid coil

now here we know that magnetic field of solenoid is along the axis of the solenoid coil and its direction is given with by right hand rule.

Now if the direction of current is reversed then as per right hand rule the magnetic field direction also get reversed.

3. dark matter

empty space could also be a simple vacuum