I have been reading Brian Greene's books The Elegant Universe and The Fabric Of The Cosmos, but got stuck with following questions:

1) In the braneworld picture matter particles and non-gravitational forces are trapped on a brane because they are formed by open strings, while gravity can move off the brane into the bulk because they are closed strings. As only the Type I string theory contains both open and closed strings, while the other four string theories (IIa, IIb, HO and HE) only contain closed strings, should these other theories not be rejected as invalid, leaving only the String I theory as a valid theory.

2) In the braneworld picture the extra dimensions can be large and even infinite. If this is the correct picture, where does that leave the Calabi-Yau manifolds, the properties of which are supposed to define the possible vibration modes, and consequently the characteristic of the elementary matter and force particles.

3) Branes are defined as ‘objects’, existing next to strings in string theory, which to me is equivalent to saying nothing, especially when in braneworlds our whole visible universe is said to be a three-brane. If branes are objects, what are they composed of?