for the first quesion, there is one theory can explain why there is gravity. it is called the General Theory of Relativity by Albert Einstein
For the second question, you dont need to worry about that. so far there is no good physics theory to describe the what happened right after the "big bang" or the universe contracted to a small point.
the current physics is in dilemma right now. before Einstein died, he was working on make one perfect physics law which can govern the whole universe, but he never suceed.
so far, we have two physic laws left to decribe the universe. and some talented physician are working toward unifying them as one theory. one promising and controversial theory is call m-string theory.
the two physics laws left are: General Theory of Relativity for describing large object such as stars and galaxies and Quantum theory for for describing small objects such as electron, neuron, quak and so on.
then you will understand. when the universe contract to a small point or exploding from a small point, we need a theory to describe both large and small object.
but unforutnately, we dont have one. and the two physic laws left are conflict with each other to applied to here.
so dont worry about it, so far no body can explain it yet.
Added after 4 minutes:
azaz104 said:
Can you help me with these two delimmas:
2-Scientists since hubbel, predicted and proved the expansion of our universe, and stepehen hawking predicted that all the universe will contract again to the single singularity it started with, but other scientists argue this and say that the the universe will expand forever, but what about the notion of SPACE, in connotation and meaning? does this mean that we are living in an infinite place? and does such thing exist?
if there is no mass, then there is no time. so the universe is expanding and also creating time. beyond that, i dont think there is anything infinity.