Dark energy, dark matter and the expansion of the Universe I have just finished reading a book which touched on dark energy and dark matter and its got me thinking. Unfortunately, I don’t have a sufficient understanding of the field to appreciate whether this is a plausible concept and would love the thoughts of an expert.
Could it be that dark matter is actually the fabric of the universe itself i.e. Space? If so, could this explain why we cannot measure it directly – if every known thing is made of ‘space’ we would have no relative medium to measure it against. 
Furthering this concept, if we were to suppose that dark energy decays to dark matter as a function of temperature, we know the universe is cooling whilst expanding at an accelerating rate, one could hypothesise that as the universe cools, the rate of decay (dark energy to dark matter) increases and hence, the rate of expansion increases.  
 A: There are a lot of reasons why it is not possible that dark matter decays to dark energy as the temperature falls. @Lawrence Crowell explained most of the important reasons, and that the two, dark matter and dark energy are tow very different things with no relation to each other. This adds, hopefully, to the reasons or why they are so. 
1) Dark matter exists in clumps, as Lawrence correctly states. Those clumps are around galaxies. Dark matter is around where there is other visible matter, and in fact it is calculated that it's gravitational attraction helped form the galaxies and stars. It is more prevalent than visible matter, by a factor of about 6-7. Total matter in the universe is about 30% of the total mass-energy of the universe, with visible matter about 4-5%, 24-25% dark matter. In the large, averaging the density of both types of matter over multiple galaxies and cluster, say over distances of about 300 million light years (thus averaging galaxies and space between them), that average or cosmological density of dark matter, and also of visible matter, is the same, or constant, throughout the observable universe. That is said as the matter, in the large, is distributed homogeneously in the universe (but it clumps in galaxies etc as explained). 
More, the total amount of matter is roughly constant in the universe, over time. As the universe expands that same total matter (including visible and dark) gets more diluted as the universe size increases. The density thus decreases, as $1/R^3$. So, yes, the density decreases, but the total quantity of matters (density X volume) stays the same. This has been observationall observed and is perfectly consistent with the standard model of cosmological expansion. Thus, dark matter has not been observed as having diminished. It could not have turned into dark energy.
2) Dark matter clumping around galaxies has been mapped. We can measure its density inside and in the halo of galaxies, and map it as very little or any at all in between them.  It has mass and gravitates (attracts other matter and energy).  It is the reason it helped form galaxies and stars. It is believed to be very weakly interacting (other than gravity) particles that were leftover remnants of the super high densities and temperatures soon after the Big Bang. They've not found the actual particles yet, they are thought to most likely be WIMPS, weakly interacting massive particles. May of them are passing through the earth and us at any time. They are looking for them. But nothing indicates they 
Are disappearing and turning into dark energy. Or viceversa, we don't see it increasing. 
3) Dark energy is covered extensively by Lawrence. It is estimated to have the same density now and in the past, and in fact the total quantity increases as the universe expands and more space, and volume, is created. It is about 70% of the universe mass-energy, the density is constant and the total dark energy is therefore increasing with the volume of the universe, I.e., as $R^3$. As it increases the rate of expansion of the universe increases - our universe's expansion is accelerating. The dark energy can not be decreasing, al,our observations and measurements are consistent with what Lawrence and I described. We know the acceleration at earlier times was less - when we look at galaxies 5 billion light years ago we are seeing what they were like 5 billion years ago. So we can see the acceleration over time, and it is consistent with the dark matter density being constant and the total increasing with the universe volume. 
4) Yes, dark energy is more mysterious than dark matter, and a toally different beast. It is as Lawrence states most likely some quantum field arising from the quantum property of spacetime, or said more intuitively, the actual energy of space. People are looking for hints of what it might be, but there's not even a good theory of quantum spacetime, or as is usually called, quantum gravity. Some quantum speculations are that it is the vacuum fluctuations of empty space. Wishing it away by having it decay to dark matter is not a possibility, it does not decay nor diminish nor turn into anything we can figure out. 
You can look in Wikipedia for dark energy and dark matter, and the universe expansion, it'll tell you more. Hopefully this helps understand the simple facts and rationale for what is described for those two dark entities. 
A: One big difference between dark energy and dark matter is their distribution in space. Dark energy is some vacuum energy that defines the cosmological constant $\Lambda~=~8\pi G\rho/c^2$, which if very small. Dark energy so far appears to be absolutely homogeneous and isotropic in distribution. It appears to be something intrinsic to the vacuum structure of the universe. Dark matter by contrast is inhomogeneous and exists in clumps. The Bullet cluster of galaxies is due to a collision of two clusters of galaxies, and the luminous matter has clumped and slowed, while the dark matter from these continues to move. The dark matter is observed by the Einstein lensing of distant sources. The Abell 520 cluster has similar signatures. 
Dark matter is then probably due to some sort of particle or field. Dark energy is most likely a manifestation of the quantum vacuum, though the details of that are not well known. There is a theory by Steinhardt called quintessence that has states the scalar field that defines the cosmological constant $\Lambda~=~\Lambda(\phi,~\partial\phi)$ is a sort of particle field that has variations. The cosmological constant has $p~=~w\rho$ for $\rho$ the vacuum energy and $p$ pressure. Currently this is thought to be have a constant $w~=~-1$, but qunitessence has
$$
w~=~\frac{p}{\rho}~=~\frac{1/2\dot Q^2~-~V(Q)}{1/2\dot Q^2~+~V(Q)},
$$
for $Q$ this quintessence field. This means the cosmological term $\Lambda$ is a dynamic variable with time and space. The time derivative is $d/ds$ is expanded in coordinate variables, which means this quintessence field is dynamical. Some interpret dark matter this way. 
Quintessence is largely not taken very strongly. Some cosmological data does not strongly support this hypothesis. One might ponder why in empty space we have $w~\simeq~-1$, but near galaxies, clusters of galaxies and halos one would then have $w~>~0$ with positive pressure that behaves as matter.
A: The fact of the matter is no body knows what dark matter and dark energy are. They are names given to the causes of two different phenomena. 
Dark matter is name originally given to the entity that causes uniform rotation curve of spiral galaxies. It also ended up explaining cluster rotations, excess gravitational lensing etc. So, it is a name used for excess gravity that can not be attributed to visible or baryonic matter. If we called it transparent matter, or invisible matter, or undetected matter, it would not make any difference. So far we do not know what it actually is. This is one thing that was not predicted by GR. In addition to being invisible/transparent, it is also non-interactive by physical contact. Only interact via gravity (until detected). That is why it has been not possible to detect it yet. It is also "cold" meaning it does not interact with EM waves and so, does not emit or absorb any radiations. These properties (being transparent, cold and non-interacting) resemble that of empty space more than anything else.
Dark Energy is name given to the entity that causes universe to expand (may be at accelerated pace in current times). Same story here - we do not know what it is. If we call it anything else, it would not change a thing. That is why some people refer to it as cosmological constant. So, this was possibly predicted by GR and was later retracted.
Because, there effects are so opposite (excess gravity vs negative gravity), it is unlikely that one would change to the other.
Dark energy is already agreed upon by most as a property of space.
Until detected and proved, there should not be any problem in considering dark matter, a state of empty space.
To avoid confusion, we need consistent naming, therefore dark matter and dark energy are the names used.
