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This question already has an answer here:

Is the total mass of the universe constant in time?

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marked as duplicate by Ben Crowell, Qmechanic Sep 15 '13 at 0:12

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

What doubt do you have? – udiboy1209 Sep 14 '13 at 16:25
I don't know why this person gets 2 down votes. Actually his question is nice.His wordings may not be right as he may be poor in English. – Rajath Krishna R Sep 14 '13 at 16:50
@RajathKrishnaR I downvoted because of the "This questions does not show any research effort" reason. Good questions put forth some effort to answer themselves and explain where the OP doesn't understand something. This makes no such effort. – tpg2114 Sep 14 '13 at 20:18

Since mass and energy are the same according to Einstein's equation I would say that mass is not constant for the universe because the law of conservation of energy has been proven false. It is so because the space itself in which the processes take place is expanding.

You can get a detailed article about it here-

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Mass is obviously not conserved, as $E=mc^2$ state. But if you want to consider the total mass of the Universe, the problem is that "total" mass (energy) in General Relativity is not well defined, see: Energy conservation in General Relativity

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