There is no concrete definition of the universe. Many of the theories that are supported by modern cosmology are backed up with evidence largely relying on the cosmological principle—the assumption that the behavior of the universe is the same elsewhere as it is here, meaning we do not occupy a location in the Universe that distorts our view of the rest of it.
Most evidence seems to support that the universe expands faster than the speed of light, meaning we cannot see the entire thing. There is in fact darkness beyond the most distant stars available to our observation. This is essentially an “emerging” portion of the universe—part that already exists, but is only now becoming visible due to the length of time it takes for the light emanating from it to reach earth.
There are models that predict earth’s locality in the universe, generally suggesting we are somewhere near the outer edge. However, because of the limitations of trying to predict the behavior of the universe using light, there is little supporting evidence to prove that what we know about the universe can accurately predict anything. The age is generally believed to be around 13.7 billion years, but these estimates are made using a number of different things—the background microwave radiation (believed to be an energy emission from the big bang), the age of extra-terrestrial materials that have landed on earth. Background radiation gives us the cooling time of the universe, and we can also observe the rate at which the edges of the universe are moving away from us.
However, these observations are based on the belief that things act no differently elsewhere in the universe as they do here, which there is zero conclusive evidence to support. Misunderstood things, such as dark matter, are believed to be faster than light, and is believed to cause the expansion of the universe, so any conclusions anyone can make about the universe is either an assumption, or a very, very rough estimate.