The observable universe refers to the portion of the entire universe that is currently visible to us given the constraints of the speed of light and the age of the universe. It represents the extent of the universe from which light has had enough time to reach us since the Big Bang.
The current estimate for the size of the observable universe is about 93 billion light-years in diameter. However, it's important to note that this figure is larger than the age of the universe (which is estimated to be around 13.8 billion years). This seeming contradiction arises due to the expansion of space over time. As the universe expands, distant objects move away from us, and light from those objects takes longer to reach us. Therefore, the observable universe is larger than the age of the universe because of the expansion during the time it took for the light to travel to us.
Now, as for what makes the universe "observable," it comes down to the finite speed of light. Light travels at a constant speed of about 299,792 kilometers per second (or roughly 186,282 miles per second) in a vacuum. Since the universe has a finite age, light emitted from distant objects needs time to reach us. Therefore, we can only observe objects that are within a certain distance from us, corresponding to the time it took for their light to reach us.
As for leaving the observable universe and entering another domain, it's currently beyond our scientific understanding. The observable universe is limited by the distance that light has traveled since the beginning of the universe. Anything beyond that distance is currently not observable to us because the light from those regions hasn't had enough time to reach us. Speculating about what lies beyond the observable universe is a topic of ongoing research and exploration in cosmology. Various theories, such as inflationary cosmology, suggest that there may be regions beyond the observable universe, but their nature and characteristics are still subjects of investigation and debate.