system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How is the universe ever expanding, i thought the universe was all that existed?
gpt
Put two pennies on a tabletop, side-by-side, just touching. 

Measure the distance from the left edge of the left penny to the right edge of the right penny. (If your unit of measurement is a penny, it'll be two pennies wide!) 

Now put a your left index finger on the left penny and your right index finger on the right penny and slide them as far apart from each as your tiny five-year old arms will allow. 

Now measure the distance  from the left edge of the left penny to the right edge of the right penny. (If your unit of measurement is a penny, it'll be a *lot more* than two pennies wide!)

The number of pennies hasn't changed, but the distance between them has. 

Similarly, the universe can expand without the amount of stuff ("all that existed") in the universe changing. What's expanding is the space between everything that exists, not the amount of stuff that exists.