If space itself is also expanding, and beyond this space, ”the black background”, nothing exists, what is the expanding space, expanding into? Science does not observe an actual expansion of space itself. All cosmological observations are based on the observations of objects. All objects are observed through the clarity of space. One could make an argument that space is not expanding and it is simply a distance increase of an object, from the observers relative point of observation, an object travelling through space and into more space. The cosmological red shift is based on an objects direction and light and not based on actual space. In context of this, there is no apparent evidence to suggest that space itself is expanding or has a finite structure. Space that does not contain visible matter has a clarity that is more see through than clear glass. For this very reason space itself is unobservable and we observe only matter that interacts with light or emits light in space. One could also argue that all experimental observations on earth of anything that is expanding , observes that anything that is expanding, needs empty space to expand into. This acceptance of Earthly physical experimental observation contradicts that there is no space beyond the last matter observed in space.
Science suggest there is nothing after this boundary, is that not similar to a flat Earth theory and suggesting we either fall off the edge of space or bump into this imaginary wall if we travelled there? yes
How can we have any certainty the universe is not finite and instead infinite? Sometimes there is logical factors that can not be simply ignored. A single answer explains this and is the only answer possible of an axiom value with no other logical options available. Infinite is without end or limit or boundary, this can be thought of in a simple logical thought experiment. At the present moment in time I presume you are indoors, you can clearly see your walls that enclose a volume of space that you are within. Outside of these walls you know that space exists and can observe space exists. You can clearly see that outside of the walls that space exists all the way to the ”black background” of space. There is a choice that only gives one answer, of what is beyond the boundary, we either live within a visual space within a space or we live inside a visual space within a solid which is within a space. There is no other physical possibilities or options and it is for a certainty that we can fit a smaller box into a larger box for a limitless amount of times.