"Deeper Space" is often part of a project called "Geometry Dash World: Beyond" or similar mod packs that try to bring the mobile spin-off levels to PC.
Since this is a custom level, you cannot simply click "download" on a random website. You need the full PC version of Geometry Dash and access to the Steam Workshop or in-game level servers. geometry dash deeper space download pc new
Once in-game, you can find the entire set of levels by searching for the List ID: 254173 in the online levels search bar. Level List and IDs "Deeper Space" is often part of a project