Vastness means a huge or immense area, such as the vastness of outer space.

Fragile Vastness was created in 2000.

The very vastness of the virgin valley that lay before us was absolutely awe-inspiring.

The vastness of space is difficult to comprehend.

Vastness means big, much like a large open space.

You use "vastness" to describe something very very large -- that can be explored.

For example, across the vastness of space lies the planet Jupiter.

The ocean is extremely vast because of how deep/large it is.

Hope this helps, cheers.