An average sandbox (whatever that is) has been said to contain as many grains of sand as the Milky Way has stars.
If every person on Earth had a sandbox, the amount of sand grains contained therin would still not approach the number of stars in the Universe.
A teaspoon contains as many grains of sand as stars you can see on a perfectly clear night.
The Milky Way is bright enough to cast your shadow on a perfectly clear night.
Most of the Universe is empty. The space between stars contains less than one atom per cubic centimeter. In contrast, at sea level, one cubic centimeter contains a billion trillion atoms.
In the time it takes you to read this sentence, the Universe will increase by 100 trillion cubic light-years.