The simple answer to your question is that if there are other universes by virtue of the expansion we see in the universe around us, there is no way we can know anything about them directly (since the distance between them is increasing, and light can only go at light speed, so no information exchange).
Now, with that said, Velenkin thought about Alan Guth's inflation that defined the initial period of the bing-bang that vastly increased the size of the universe, and asked the question why did this inflation stop? The conclusion he came to was startling - there were other universes existing next to ours, stopping ours from further inflating! Suggesting other universes that we could know nothing about meant that Velenkin's idea was met with great hostility.
Similarly, super-string theorists trying to see mass and massless particles as strings could only make their theory work if there were many dimensions, certainly more than 3 spacial and 1 time dimensions. That raised the question why do we have 3+1 dimensions rather than the extra ones string theory suggests? The answer they came to was also startling. Several distinct universes could co-exist in proximity separated by at least 1 dimension, and such an arrangement would influence which, and how many, dimensions grew and gained dominance within a universe.
Although there is no direct evidence for multiple universes, when two or three separate, apparently unrelated theories start pointing in the same direction, it makes the circumstantial case quite appealing to physicists.
With that said, we know that space and time are related because Einstein's relativity has shown that relativistic effects affect both. For example a clock at rest will advance rapidly, as will one far away from a gravitational body. However, a clock moving rapidly relative to the speed of light will advance more slowly, almost as if that clock can either traverse time quickly or space, but not both. There is a relationship between space and time. For example, as you accelerate this same clock near the speed of light and time slows, its distance (or length) will also compress. For this reason a Cartesian view of space ceded to a more dynamic relativistic view of space after Einstein, and people began to refer to it as 'space/time'.
Therefore, if the big-bang provides evidence that all matter began to exist at some point, so did time, since space and time are related. What existed before time 0 is not a scientific question but a metaphysical one, since science typically restricts itself to what is within space/time, and studies things that are testable using observation, and repeatability (which is not to say that we cannot actually reason about metaphysical things).