Re: Is Dark Energy Internal or External to our Universe?

This is a difficult question to give a definite answer to.  Let's begin by
thinking a bit about the philosophy of science. Many scientific thinkers,
particularly over the last 150 years or so, have subscribed to an empirical
or "positivist" view of science -- roughly speaking observations are
everything and there is no sense in which one can delve deeper than what
can be observed.  This is certainly the underlying foundation of the
Copenhagen interpretation of quantum mechanics, for instance.

Others, such as Einstein particularly in his later life, have preferred a
more "realist" attitude, believing that there is a true reality which
underlies the world, and that our observations only give us clues as to
what that might be.

I think it's safe to say, though, that both camps would generally be
uncomfortable about postulating the existence of scientific entities and
structures which go beyond what can be observed in principle.  One thinks
of Newton's ideas of absolute space and time, which existed independently
of the material world, and was only invented to provide structure to the
underlying theory, but couldn't be observed or influenced in any way. This
concept caused much controversy, ultimately leading to the development of
relativity theory.  Interestingly, one way of considering the space-time
curvature of relativity theory is to imagine the curvature taking place in
some higher dimensional universe, however this was never regarded as a
logical part of the theory.  It doesn't mean that there can't be any such
extra dimensions, it's just that the extra dimensions would be only
introduced into the theory to make it easier to picture in our minds, and
wouldn't have any other explanatory power.  In such a situation it is hard
to say whether something with which we can never interact can be considered
"real".

Nonetheless, I would argue that in modern science we do meet situations
where speculation about theoretical structures beyond or outside our
universe are either tolerated or accepted.  Certainly in cosmology the
basic theoretical models we use extend beyond our observable universe. 
More radically, there have been many efforts to consider how these models
may be too simple, and that a more "realistic" theory might give rise to
"multiverses" which only connect to our universe beyond the horizon we can
observe.

Another example might be the "many worlds" interpretation of quantum
mechanics, which postulates the existence of an uncountably vast number of
parallel universes which don't interact with ours, but which are
continually being created.  This idea is not demanded by the basic theory
of quantum mechanics, but many feel that it provides a more elegant
explanation of its more bizarre aspects.

All this brings us back to your question of whether dark energy could be
the product of a force from "outside our universe".  In light of what we've
considered above, the first thing to ask is whether such an idea makes any
sense?  Certainly there's no reason why  it is logically impossible.  If
the source of the dark energy in such an "external" model led to
observational differences compared to standard theories, then it becomes a
legitimate scientific concept, which may be disproved or found to be a good
description of the world.  One might even say that this "external space"
would ultimately have to be considered part of our universe even if our
material selves could not travel in it.

Interestingly, in the last few years, an influential, but still
speculative, "industry" has been spawned which takes up the
ideas of superstring theory to suggest that our universe may indeed be just
a slice (or membrane, or "brane") in a larger dimensional space.  In this
"braneworld" framework most particles and forces can only exist on the
brane, but gravity can leak out.  So this already begins to sound rather
like what you propose, although it doesn't really explain dark energy in
the way you suggest.

So, broadly the answer to your question could be "why not?", if such a
theory could be formulated mathematically.  However, we do need to be
careful in understanding what we seek to explain here.  You say that the
"universe accelerates faster towards its outer limits", but the universe we
see appears to be completely symmetrical, with no indication of any outer
limits (other than the horizon all around us).  The acceleration appears to
be increasing in time, but as far as we can tell is doing this in a steady
way everywhere in space.  Similarly, the evidence suggests that the Big
Bang singularity did not explode "consuming space", since the space did not
pre-exist the Big Bang, and indeed (by symmetry) immediately following the
Big Bang space was uniformly filled with (very rapidly expanding, or
perhaps better to say, diluting) matter and energy.  

In conclusion, it's certainly fair-game to consider physical theories which
go beyond our current understanding of the limits of our universe, and if
that leads to observationally testable predictions then it is very much
"legitimate" science (otherwise it might be considered meta-physics).  The
big problem, unfortunately, is formulating any such new ideas in a
mathematically rigorous way, which agrees with what we already know about
the universe -- and that ain't easy!