text: There is no definite answer to your question, since science has a limit to how small particles it can detect.  The basic theory in this area is that matter is just a standing wave.  A wave has no weight, but when encased in a sphere, it then has mass.  And mass requires weight.  This weight is so infinitesmally small it cannot be detected.  But these spheres, called photons, are held together with other photons making up quarks, which are held together by Glueon, which in turn makes up atoms, etc.  This is all based on the wave/particle duallity of light.  For more info on this, just e-mail me at msm0021@hotmail.com.