I would propose the material currently missing from our universe was all there at the Big Bang moment but over the billions of years much has radiated off into the void. Every star,pulsar,quasar and black hole radiate out, from birth, their contents over time. Some of the material would be retained but much would be lost especially at the outer fringe of the universe where there is nothing to stop it from leaving the universe entirely.
As an additional thought, is it possible that the expansion of the universe is caused by the pressure of all the radiation flowing out from every body onto every other body. I remember the ECHO satellite being pushed out of its orbit by sunlight.
Any reader will soon determine that I am a rank amateur despite my 77 years.