The universe is generally understood to be everything which physically exists, including all forms of matter and energy and all events that occur involving the two. It is sometimes contrasted with a supposed "other world" of some metaphysical nature (e.g., other "planes of existence").
According to the most widely accepted scientific theory, the universe consists of a four-dimensional "continuum" of space-time (three dimensions of space and one of time) that grew out of a singularity (a dimensionless point) in an event called the Big Bang about 12.7 billion years ago.
It is currently unknown whether the universe extends beyond the observable universe, that part which we are able to observe (at least theoretically) from our particular vantage point at this particular time (because of the age of the universe, this portion would be about 12.7 billion light years across). Many cosmologists assume that there are portions of the universe which we can not (presently) see.