Nowhere is an empty white void which was visited by Squidward Tentacles in "SB-129." After being chased out of the past by Primitive Sponge and Primitive Star, the time machine he travels goes haywire, after he broke the switch, and transports him to this white wasteland of loneliness. He seemed happy to not be with SpongeBob and Patrick, but he quickly realized how lonely this place was and only escaped from here because he begged the time machine to take him to home after admitting he missed SpongeBob as well as his Easter Island head and Bikini Bottom. Apparently, it is right above the time machine.
From first glance, there appears to be nothing but a white screen at first, but walking down a bit further reveals some different colored squares that can be lifted up, and the squares can be seen on the floor and ceiling. Also, there are some strange voices that can be heard and every sound heard is echoed, that also includes the sound of one's voice and those same odd voices seem to repeat what one says (e.g., when Squidward said "Alone!", many other voices echoed back "Alone...") and if that wasn't enough, there are speech bubbles that seem to pop out of nowhere. Besides the squares, there seems to be absolutely nothing else at all, nothing physical, no trace of life, just plain emptiness that seems to go on forever (when Squidward tried running through it, he kept looping back to where he started). An interesting thing to note is that it seems breathable for creatures who normally need water in order to breath, an obvious example being Squidward, when there doesn't seem to be any water at all. There seems to be no exit out of this place of eternal loneliness (the only reason Squidward escaped is because he begged the time machine to take him home and it was lucky for him the machine was under his feet, perhaps the entire time). It can be assumed absolutely nothing or nobody is here because it is the end of time.
When Squidward first entered here, he seemed to be the only thing visible in this lonely place. As he walked along further, he found square tiles that he could lift up. After summing up that SpongeBob and Patrick were nowhere within sight, he relished in the fact that he found a place where he could be all to himself and with nobody else around (in the background there is a pink square and a yellow square that may resemble SpongeBob and Patrick). However, after he heard many voices echo "Alone...", he realized the eternal loneliness of this place, and then proceeded to get out by running. That proved futile because Squidward keep ending up right back to where he started from. After looping back 4 times, Squidward notices the time machine's is nowhere, and wonders where anything is. After stomping the floor 4 times (as well as shouting "Where?!" 4 times), he found the time machine again after falling through its ceiling. Squidward then finally broke down, begged to go home, and admitted he missed SpongeBob. After he spoke the final sentence, Squidward was finally able to escape the realm.