Experimental elementary particle physics is collecting data in order to give answers to many questions of both fundamental and existential nature. We already know that all forces are associated with particles and exchange of force mediators. Thus it is very important to understand and to explain the most fundamental concepts such as mass, spin and charge. Controlled experiments can reveal the particle contents of the Universe, and tell us how this has influenced the shaping of the world from the Big Bang and possibly into the far future. The seemingly different forces of nature appear to have a close relationship which can be revealed by new collider experiments. The results point to a unified picture where forces that we are used to regard as weak become strong at extremely high energies (temperatures) with consequences which go far beyond our daily experience but which still prevail in parts of the Universe.

The experimentally well tested Standard Model describing electroweak and strong interactions is in many respects not a complete theory, as it does not answer to our most fundamental questions about how particles come about, sizes and stability of particles. Even our most common stable particles such as the electron, proton, photon and neutrino are not explained and are therefore still investigated in numerous experiments, e.g. at CERN and DESY:

• how small is the electron and what forms the unit charge?
• how come that the proton which is composed by quarks is stable?
• what mixture of virtual electrons, quarks and gluons is there inside the photon?
• what are the masses of the neutrinos and do they oscillate?

New experiments using LEP and LHC at CERN are aimed at closing up on the so far hidden secrets of matter by looking for Higgs- and SUSY-particles which might give us the clue to the next big step in understanding the world around us.