There is antimatter, but "a giant cloud" is very much the wrong description for it. So let's clear up some possible misconceptions.
Antimatter is not that unusual to find...
Sure, it annihilates rather quickly when it interacts with normal matter, but space tends to be rather, well, empty. So if you make an antimatter particle, it might be some time before it can find something to annihilate with. Moreover, plenty of normal matter begets antimatter: radioactive nuclei with too high a positron/neutron fraction can undergo positron emission, where indeed they emit an antimatter particle.
...But it should be a relatively small fraction of total matter.
The "cloud" mentioned is a region with more antimatter than you might naively expect, but not overwhelmingly more. It is a large region with thousands of star systems, all behaving normally, and, as I'll calculate below, consists almost entirely of normal matter, even excluding the stars.
Tracing the news to its source.
First, we should find the original scientific publications here. The story you linked was actually based on this 2008 NASA press release, which was in turn written about the 2008 Nature article "An asymmetric distribution of positrons in the Galactic disk revealed by $\gamma$-rays".
Of note is that this "cloud" is old news - the abnormal antimatter signal was seen decades ago. All this paper does is note that the signal is coming from a region slightly off-center from the center of the galaxy, which might provide a clue to the source (more about that later).
How much antimatter is there really?
The Nature article gives us some important facts. First, the antimatter signal of interest is the $511\ \mathrm{keV}$ line. This is the result of a positron ($\mathrm{e}^+$) annihilating with an electron ($\mathrm{e}^-$), producing two high-energy gamma-rays (photons, $\gamma$):
$$ \mathrm{e}^+ + \mathrm{e}^- \to 2\gamma. $$
Both gamma-rays have an energy of exactly $511\ \mathrm{keV}$, as can be shown from simple conservation of energy and momentum. Thus we know the antimatter is specifically some wayward positrons (anti-electrons), not whole atoms and molecules and such.
They say the size of the emission region is about $600\ \mathrm{pc}$ across,1 and so we'll take it's radius to be $R = 300\ \mathrm{pc} = 9\times10^{17}\ \mathrm{cm}$ They also tell us they see a flux of $F = 1\times10^{-3}\ \mathrm{cm^{-2} s^{-1}}$ (photons per square centimeter per second). Furthermore, they cite another article, "Spectral analysis of the Galactic $\mathrm{e}^+\mathrm{e}^-$ annihilation emission," which modeled the interstellar medium and concluded that in those conditions the average positron would wander for some time $\tau = 10^5\ \mathrm{yr} = 3\times10^{12}\ \mathrm{s}$.
Let $n$ be the number density of positrons in the "cloud." Now a volume $V = (4\pi/3) R^3$ will have $nV$ positrons in it, and there will be a gamma-ray production rate of $Q = 2nV/\tau$. If $A = \pi R^2$ is the projected area of the cloud, we expect to observe a flux of $F = Q/A = 8Rn/3\tau$. Thus we can calculate
$$ n = \frac{3\tau F}{8R} \approx 1\times10^{-9}\ \mathrm{cm^{-3}}. $$
The typical number density of hydrogen in interstellar space is something like $1\ \mathrm{cm^{-3}}$, so the positrons are about one part per billion of the interstellar material in this region.
Where are the positrons coming from?
The excitement in these articles is the asymmetric placement of the cloud, as alluded to earlier. In particular, the Nature article claims this asymmetry is the same as that observed in the distribution of X-ray binaries near the center of the galaxy. Now X-ray binaries are stellar-mass black holes that are accreting material from a companion star, resulting in a hot accretion disk and maybe some relativistic jets pointing along the axis of the disk. Such extreme conditions can produce positrons, and these can be blown out into interstellar space, where they persist for a hundred thousand years or so.
Alternative sources are known. For instance, ${}^{26}\mathrm{Al}$ is a radioactive isotope produced in some supernovae with a half-life of a bit under a million years. One way for it to decay is through positron emission:
$$ {}^{26}\mathrm{Al} \to {}^{26}\mathrm{Mg}^* + \mathrm{e}^+, $$
where the nuclear excited state ${}^{26}\mathrm{Mg}^*$ decays to the ground state ${}^{26}\mathrm{Mg}$ by emitting a $1809\ \mathrm{keV}$ photon. Indeed, the Nature article discusses removing part of the $511\ \mathrm{keV}$ signal based on what the $1809\ \mathrm{keV}$ line is telling us about the ${}^{26}\mathrm{Al}$ contribution to the positron population.
The other intriguing idea that has been discussed is that the observed gamma-ray excess is related to dark matter annihilation. That is, the current best guess we have for the nature of dark matter is that is is a species of particle that very rarely interacts with anything else, but can occasionally annihilate with itself, producing photons or possibly other particles like positrons.
In summary...
There are more positrons at the center of the galaxy than elsewhere relative to normal matter, but they are still a miniscule part of the interstellar medium. If there were an area consisting mostly of antimatter, or about equal parts matter and antimatter, either its surface or its interior would be incredibly bright, far brighter than any signal we detect.
1 Well, actually they say it has an angular full width at half maximum of $6^\circ$, and that at that distance $1^\circ$ corresponds to $100\ \mathrm{pc}$.
Black holes cannot be seen because they do not emit visible light or any electromagnetic radiation.
This is not absolutely correct in the sense that visible light is emitted during the capture of charged matter from the radiation as it is falling into the strong gravitational potential of the black hole, but it is not strong enough to characterize a discovery of a black hole. X rays are also emitted if the acceleration of the charged particles if high, as is expected by a black hole attractive sink.
The suspicion of the existence of a black hole comes from kinematic irregularities in orbits. For example:
Doppler studies of this blue supergiant in Cygnus indicate a period of 5.6 days in orbit around an unseen companion.
.....
An x-ray source was discovered in the constellation Cygnus in 1972 (Cygnus X-1). X-ray sources are candidates for black holes because matter streaming into black holes will be ionized and greatly accelerated, producing x-rays.
A blue supergiant star, about 25 times the mass of the sun, was found which is apparently orbiting about the x-ray source. So something massive but non-luminous is there (neutron star or black hole).
Doppler studies of the blue supergiant indicate a revolution period of 5.6 days about the dark object. Using the period plus spectral measurements of the visible companion's orbital speed leads to a calculated system mass of about 35 solar masses. The calculated mass of the dark object is 8-10 solar masses; much too massive to be a neutron star which has a limit of about 3 solar masses - hence black hole.
This is of course not a proof of a black hole - but it convinces most astronomers.
Further evidence that strengthens the case for the unseen object being a black hole is the emission of X-rays from its location, an indication of temperatures in the millions of Kelvins. This X-ray source exhibits rapid variations, with time scales on the order of a millisecond. This suggests a source not larger than a light-millisecond or 300 km, so it is very compact. The only possibilities that we know that would place that much matter in such a small volume are black holes and neutron stars, and the consensus is that neutron stars can't be more massive than about 3 solar masses.
From frequently asked questions, What evidence do we have for the existence of black holes?, first in a Google search:
Astronomers have found convincing evidence for a supermassive black hole in the center of our own Milky Way galaxy, the galaxy NGC 4258, the giant elliptical galaxy M87, and several others. Scientists verified the existence of the black holes by studying the speed of the clouds of gas orbiting those regions. In 1994, Hubble Space Telescope data measured the mass of an unseen object at the center of M87. Based on the motion of the material whirling about the center, the object is estimated to be about 3 billion times the mass of our Sun and appears to be concentrated into a space smaller than our solar system.
Again, it is only a black hole that fits these data in our general relativity model of the universe.
So the evidence for our galaxy is based on kinematic behavior of the stars and star systems at the center of our galaxy.
Best Answer
The man who catalogued the most dark nebulae was E. E. Barnard, the same person who discovered Barnard's Star. These nebulae are known by the numbers in Barnard's catalog, such as B.33, the Horsehead Nebula.
Barnard was one of the first people to apply photography to astronomy, and one result was a stunningly illustrated volume of dark nebulae.