Why does energy conservation fail on large scales?

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Or is it absolute? I've heard the conservation of energy on a large scale doesn't hold true. How can this be so?
 

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Energy is relative, both in Relativity and in Newtonian mechanics. This means that different reference frames will determine different values for the energy of a given object at a given point in space and time. However, the conservation of energy holds in any reference frame. This means that, in a single reference frame the total energy (KE + PE) will be constant over time.

This is the difference between the concepts of "invariant" and "conserved". Energy is conserved, but not invariant.
 
In General relativity, for technical reasons, the conserved quantity is called the stress-energy pseudotensor, rather than the stress-energy tensor. This is why you may have heard that energy is not globally conserved. As DaleSpam indicated, the stress-energy pseudotensor is globally conserved, but it is coordinate dependent.

Edmund Bertshinger's notes:
http://web.mit.edu/edbert/GR/gr7.pdf
 
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