Virtual particles

Virtual particles are particles described by quantum physics that exist for an extremely limited space and time. The term virtual should not confuse you into thinking that these particles do not exist. They really do exist, interacting with other particles, producing a measurable effect on their surroundings. The very laws of physics prevent them from ever actually being directly seen or measured.

Virtual particles do have mass, even when they are part of massless forms, such as photons.

How?
The vacuum of space (or, more correctly any "space") has an energy level. Nothingness is in fact something. Due to the uncertainty principle, virtual particles will always appear from the energy of a vacuum and always appear in pairs. These particles "borrow" energy from the vacuum and immediately collide and annihilate themselves, repaying the energy back into the vacuum and thereby do not violate the laws of thermodynamics. This process has implications for the development and eventual dissipation of black holes; when a virtual pair appears next to the event horizon of a black hole, one particle may fall in and if that happens the other will free itself. In order to maintain the first law of thermodynamics (energy cannot be created or destroyed) the black hole must then give up a little of its own energy to repay the lost energy - this is called "Hawking radiation."