[ARVIND (OP)]

        According to the laws of conservation of mass, mass can neither be created nor be destroyed. But, according to Einstein`s famous formula E=mc² , mass can be converted to energy. So, does this this mean that the quantity of mass and energy in the whole universe is in inverse proportion. 

[evan_au]

For experiments that we can conduct in the laboratory today, an inverse relation is found to exist between energy and mass.

This belief is so strong that when some experiments found that mass/energy/momentum was "missing", it led to the successful modelling of the neutrino, an elusive particle which is very hard to detect.
Scientists working on the Large Hadron Collider are continuing to look for missing mass/energy, as it could again indicate the existence of even more subtle particles - perhaps those comprising the mysterious "Dark Matter".

The Big Bang theory suggests that the universe was initially very hot and dense, and then some of this energy turned into the matter we see today. It is thought that the visible matter we see consists of about 10⁸⁰ protons+neutrons (plus a similar number of electrons as the protons). It is thought that a larger fraction of this initial energy turned into Dark Matter - but we don't really know what this is, yet.

An even larger amount of energy is thought to reside in Dark Energy, driving the expansion of the universe. It's a bit early to say whether this can be converted into mass and back again, or whether it will just decay as the universe expands.

The amount of mass in the universe is not expected to remain constant over time. Some theories suggest that over extremely long periods of time, matter will again decay into energy, but its density will be so low that it won't form new matter. http://en.wikipedia.org/wiki/Future_of_an_expanding_universe#Dark_Era_and_Photon_Age

[yor_on]

Don't know. Do the universe exist as a 'whole unified global description', or is it all local descriptions creating the global description to us? Does a black hole exist in 'our universe', or not? Does quantum mechanical fluctuations of energy have a finite amount, as in 'zero point energy', or can one assume that there are no limits?

[Ethos_]

Don't know. Do the universe exist as a 'whole unified global description', or is it all local descriptions creating the global description to us? Does a black hole exist in 'our universe', or not? Does quantum mechanical fluctuations of energy have a finite amount, as in 'zero point energy', or can one assume that there are no limits?

As Yor_on has suggested, while this question is an interesting one, it's answer will not be easily defined. It's very possible that black holes are exit ways for matter and energy from our universe to another. Given the recent theory about brane collisions also causing the Big Bang and what-not, the variables contained within this question are staggering.

As a side to this question, something tells me that the precise volume of our space/time capsule has a definite relationship to this answer.