I thought that deep space was 100% empty of matter, except for the random star, nebula, black hole, or planetary system. Since I always heard that space was a vacuum, I also believed that except for the dust and dirt inside a vacuum bag, I believed that the rest of the "space" in a vacuum bag was totally empty, just like space. I learned otherwise about the vacuum bag pretty early on, but I finally learned that outer space is not completely empty some time in 8th grade Earth Science.
A childhood belief about physics

others believed things like this
- i used to think that vacuum cleaners had little piece of space inside them because space is a vacuumAnon
- I totally believed my siblings when they told me the family was aliens and had a space ship in the garage.— cully
- When I was really little I used to belief that fireworks dropped out of the sky from outer space.— davesgrl2001
- I thought the airport was a place that you get chopped into pieces in order to go to outer space! lol!!!— I don't know why you get chopped...
- when I was little i was scared of vacuum cleaners. I thought they would suck me inside of them.— Julie
- I used to think that up in outer space the sun was a gigantic baby that never grew up like in Teletubbies.— WeirdButProud