Virtual Photon.

It was I believe first suggested by Feynman in his quantum electrodynamics. Here the idea is that the force between two charged particles needed a mediator. This mediator is supposed to be virtual photon. It is called virtual for the simple reason that it doesnot exist as such that is it a beautiful approach to explain the attraction between these particles.
The fact is the particle according to this theory sends a virtual photon having wavelength proportional to the energy it has. This means that Higher the energy shorter the wavelength and so it can delve into the nucleus of an atom. That is only if the wavelength is small enough it can interact with other particles of that size.
This is one of the reason why Particle accelerators get bigger and bigger. Latest particle accelerators accelerates one electron upto Tera Electron Volt ranges. So the corresponding virtual photons wavelength will be so small. Presenting new evidence to the scientists.

Now all these mediator particles like photon, graviton gluon(mediator of strong force, coz its like glue farther apart stronger the force.) are included in the family of particles gauge bosons. Basically it is like two dogs for one bone but it is an oversimplification.