What ultimately happens to the bulk of matter in any trophic level of a biomass pyramid that is the matter that doesnt get passed to the trophic level above?

The organisms are made up of matter. Some of the matter is transferred from one level to another and the left material undergoes the decomposition process after the death of the organism.  

The Decomposers and the detritivores convert the fixes the organic matter into the soil. This way, the dead remains converted into inorganic nutrients, which can be used by the producers again.  

sorry, but you do not have posted the diagram.