Does sluggish decoherence in deep space have any noticeable effects? It is well established that quantum mechanical systems lose their coherence over time due to interaction with an uncontrollable environment. In particular, Professor Haroche, among others, has experimentally measured decoherence rates in mesoscopic quantum systems such as photons in a cavity and verified the theoretical predictions. 
Now suppose we are in deep space, where energy density and temperature is much lower than on earth. Current theory would say that the decoherence rate is much slower. Does this "slowing down" of decoherence have any noncontrived macroscopic effects, or any influence on the structure of matter in deep space vs. on earth? 
 A: The answer is it depends. In deep space you're in a radiation bath with a temperature of about $2.7\operatorname{K}$ called the comic microwave background. That's warmer than the superfluid transition temperature of Helium-4. So we routinely make pockets here on earth that are far colder. So any effects that can be decohered by being at a temperature that high won't happen
One example of a unique effect you get, due to the extremely low density of particles, is a reduction of collisions between atoms, and that buys you the time needed for lone neutral hydrogen atoms to emit light at $21 \operatorname{cm}$. You can also see spectral lines from what are known on the ground as "forbidden" transitions (for example, in doubley ionized oxygen) because in a denser gas collisions between ions will drain the energy from an excited state before the photon can be emitted.
It is debatable, though, whether it is accurate to describe such phenomena as coherence since they're more a question of available decay time between a particular type of collision.
A more unambiguous example of an area where outer space produces coherent quantum phenomena is in the area of ammonia and water masers (example observation from the literature).
I don't know if sluggish decoherence for mesoscopic systems (dist grains with size about $1\operatorname{\mu m}$) is possible out in the space between galaxies, but I'm pretty sure that any matter that gets ejected back out to those regions from galaxies get atomized (literally) in the process. The places where dust grains like this exist are much lower gas pressure than we can make, but also a good deal higher temperature: 

The cores of molecular clouds are the coldest regions of our Galaxy, with temperatures in the 8°K to 20°K range.  These cores are also the densest regions of the interstellar medium, with the number of molecules exceeding 105 per cubic centimeter.

There is, also, an interesting quantum effect that caused a bit of a controversy 60 years ago or so. Photons that started, almost certainty, from different parts of a star become correlated on their trip to Earth in a process called the Hanbury Brown and Twiss effect. The argument that this disproved quantum mechanic lead Purcell to write:

Moreover, the Brown-Twiss effect, far from requiring a revision of quantum mechanics, is a instructive illustration of its elementary principles.

