Riddle solve plz

Who am I?
The light can not travel through me
The length of me can not be measured
The lamps can not brighten me
The limp can not slow me
The man can not see me
The boy can not touch me
The girl can not hear me
The student can not guess me
The teacher can not write about me
The optimists can not be optimistic
The geniuses have got no clue
Can you find the answer?