Uncontrolled airspace (also known as 'class G' airspace) exists close to the ground where radar cover is not possible. Air traffic control does not exercise any authority in uncontrolled airspace.
Because IFR flight is not possible in uncontrolled airspace (due to ground proximity and lack of radar coverage), VFR is permitted even in extremely poor weather. VFR craft need only remain clear of clouds as there is no risk of colliding with an IFR craft emerging from that cloud. Daytime flight is permitted even if visibility is just one mile.
A common misconception is that all aircraft in controlled airspace are under the direction of air traffic control. In fact, most controlled airspace is 'class E' and aircraft are not required to have a radio to fly in this airspace.