Is there any phenomenon where a supernova doesn't anything leave behind? I have been researching this on google, and I think there is a process. What is this process called? I have forgotten.
You are probably thinking of a pair-instability supernova. Such a supernova releases more energy than the star's gravitational binding energy, completely disrupting the star and dispersing the contents.  Meaning that with nothing else in the universe, over infinite time every particle from the star will get infinitely far away from any/every other particle.  The mass, energy, etc. of the star doesn't vanish from the universe, though; it just gets spread throughout it.  But there will be no dense remnant left behind: no dwarf, no neutron star, no black hole; over long time scales there won't even be a discernible nebula remaining (though note the star may have shed off significant amounts of material prior to this event, which would interact with the ejecta and may result in some sort of permanent structure suggesting a star used to be there).