Nobody knows to date if there is a boundary exists & what is 'outside' the universe. Scientists only know that the observable universe is expanding at a fast pace.

First, you must differentiate 'observable universe' & universe (whole) itself.

Our science today can only give a rough figure of the observable universe at around 14 billion parsecs radius, making it a diameter of about 28 billion parsecs. Therefore no one can accurately tell you the size of the universe, that is if we assume the universe is finite (has an outside, in layman term). However there are scientists gauge the lower boundary of the universe diameter size at about 76 billion parsecs, assuming the universe is finite & various cosmic variables.

So even the universe size itself we humans are unsure of, let alone what is outside the universe. Hence we can only speculate & guess. One particular popular example is outside the universe is another 'universe-like space' filled with multiverses (parallel universes), however this will spread a lot of debate whether if parallel timeline is possible in the first place.

Try to read more articles on the net, there is a lot of discussion & information about it.