How is a photon converted into matter? Photon is a mass less particle but still it can be converted into matter which has mass. 
I know the formula $E=mc^2$, but is there some kind of force or particle which converts photons into matter, is it the weak nuclear force? 
 A: The conversion of a pair of photons into an electron/positron pair is simply the time reversal of the anhilation of an electron/positron pair into a pair of photons. The thing that permits the transition is conservation. Specifically, all of the conservation laws are satisfied including energy, momentum, spin, charge, etc. Regardless of which fundamental forces are involved, there is a certain probability of the transition as long as all of the conservation laws are satisfied. 
A: In fact photons are "messengers" (Gauge Bosons) of the Electro-Magnetic forces (it's a fundamental interaction). If you use (use could be "convert") photons it rely on the EM if I'm not mistaken. 
There isn't a mysterious force which convert the messengers of a fundamental force into mater. It's simply authorized by the conservations laws.
https://en.wikipedia.org/wiki/Breit%E2%80%93Wheeler_process :
The inverse process of e- e+ annihilation is still complicated. We have no laser sufficiently powerfull to create a pair with photons. The probability of this kind of event is very low.
A: You might want to consider the formula $E^2-p^2=m^2$, where $E,p,m$ represent energy, momentum, and mass, respectively.  Deriving this is a bit tricky if you don’t know the proper formulas for energy and momentum; check out this link for more.  
On a side note, just as a neat thing most people who don’t study this stuff in-depth don’t know:  $E^2$ does not equal $mc^2$ exactly, especially at relativistic speeds.  The actual equation is $E= \frac{mc^2}{\sqrt{1-\frac{u^2}{c^2}}}$. That might actually be $E^2$ not $E$; I can’t quite remember and don’t have my notes with me.
