You are effectively asking 'what is beyond all that is known'. A shaman would roll his eyes and simply respond 'the unknown'. A scientist would shrug and say 'nothing'. While that sounds pretty vacuous, it captures the essence of the question. You cannot picture the universe from outside the universe because everywhere and anywhere possible to go is, by definition, part of this universe. The only known constraint is called the particle horizon - the distance photons have traveled since the beginning of time and that 'boundary' continues to recede from us at light speed. Distance, however, is always relative to something and that something is the big bang: which turns out to be singularly unhelpful. The big bang occurred everywhere - not at some uniquely identifiable location.