As technology has evolved, astronomers are able to look back in time to the moments just after the Big Bang. This might seem to imply that the entire universe lies within our view. But the size of the universe depends on a number of things, including its shape and expansion. Just how big is the universe? The truth is, scientists can't put a number on it.
The observable universe
Astronomers have measured the age of the universe to be approximately 13.8 billion years old. Because of the connection between distance and the speed of light, this means they can look at a region of space that lies 13.8 billion light-years away. Like a ship in the empty ocean, astronomers on Earth can turn their telescopes to peer 13.8 billion light-years in every direction, which puts Earth inside of an observable sphere with a radius of 13.8 billion light-years. The word "observable" is key; the sphere limits what scientists can see but not what is there.
But though the sphere appears almost 28 billion light-years in diameter, it is far larger. Scientists know that the universe is expanding. Thus, while scientists might see a spot that lay 13.8 billion light-years from Earth at the time of the Big Bang, the universe has continued to expand over its lifetime. Today, that same spot is 46 billion light-years away, making the diameter of the observable universe a sphere around 92 billion light-years. [VIDEO: Oldest Light in the Universe: How it Traveled to Us]
Centering a sphere on Earth's location in space might seem to put mankind in the center of the universe. However, like that same ship in the ocean, we cannot tell where we lie in the enormous span of the universe. Just because we cannot see land does not mean we are in the center of the ocean; just because we cannot see the edge of the universe does not mean we lie in the center of the universe.
The shape of the universe
The size of the universe depends a great deal on its shape. Scientists have predicted the possibility that the universe might be closed like a sphere, infinite and negatively curved like a saddle, or flat and infinite.
A finite universe has a finite size that can be measured; this would be the case in a closed spherical universe. But an infinite universe has no size by definition.
According to NASA, scientists know that the universe is flat with only about a 0.4 percent margin of error (as of 2013). A flat universe is an infinite universe; thus the size of the universe is infinite.