String

String is the main component of the string theory; one of the most popular quantum particle theories ever. String theoretically are one of the most unstable things, and they "vibrate" certain frequencies that emit particles from higher dimensions into our local dimension, all the way up into the 11th dimension. String has only been hypothesized and has not yet been confirmed if it's real or not. However, if it does exist and we were to figure it out in the near future and study it, it would explain the entirety of the universe, it's laws, and everything else. This would also explain the fact that smaller particles cannot possibly exist (other than the Planck Length).

All the verses beyond String are speculation, but they may exist in some form if a new hypothesis is developed.

Higher Dimensional
String is theoretically higher dimensional. At least, it is in superstring theory and m theory. Basically, string is attached onto a higher dimensional (2d) brane, which is attached onto a 3d bulk, which is attached to a 4d bulk, etc until the 11th dimension. Hypothetically, going past the 11th dimension isn't possible due to the fact that going past the 11th dimension would require multiple "everythings", as the 11th dimension is theoretically absolutely everything possible at any point in time in any alternate timeline. The 11th dimension transmits to the 10th, which transmits to the 9th, etc until they reach our dimension, and hypothetically even lower.