As of now, string theorists have no explanation of why there are three large dimensions as well as time, and the other dimensions are microscopic. Proposals about that have been all over the map.
-- Edward Witten
   
Edward Witten   Name:
Edward Witten
Type:
Mathematician
Nationality:
American
Birth:
August 26, 1951
https://www.nutquote.com/quote/Edward_Witten