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Being said that the antimatter - matter reaction is faster than that of a fission and fusion, what if the antimatter cloud found at the center of our galaxy could really able to react with matter from the stars around it and cause a chain reaction of annihilation that spreads through the galaxy?

Waffle's Crazy Peanut
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Ken
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    what antimatter in the center of our galaxy? The hypothrsis is for a blck hole, not antimatter. http://en.wikipedia.org/wiki/Galactic_Center . antimatter/matter interact with the strong force, as does matter matter, at the similar rates, and will not make a difference for a black hole. Chain reactions need density of matter and even if there existed an antimatter kernel there would be no chain reaction since space is mostly empty as far as the size of elementary particles goes. – anna v May 13 '13 at 14:27
  • http://www.nasa.gov/centers/goddard/news/topstory/2007/antimatter_binary.html – Ken May 13 '13 at 14:29
  • I know the space is empty! Comparing the gigantic extent of the antimatter cloud how could it not react with matters from stars surrounding it (or the one engulfed by it)... Just like a gigantic Antimatter Bomb! – Ken May 13 '13 at 14:31
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    The link & the question is good for sure. But, space isn't so empty at all (addressed by many questions already asked here) – Waffle's Crazy Peanut May 13 '13 at 14:36
  • to make a bomb you need high densities. They use implosion, for example, for bombs to initiate fusion chain reaction. What you are seeing are normal annihilations when matter, which is the majority, meets antimatter. No density is given in the link you provided, which should be in the main body of the question, but the word "cloud" more or less covers that it is nothing dense. – anna v May 13 '13 at 14:51
  • an postiron meets an electron and annihilates into photons. It cannot go on to create more annihilations in bulk to generate a chain reaction. They have not seen antiprotons either. – anna v May 13 '13 at 14:58
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    This is not a cloud of antimatter, despite what the press releases would have you believe. It is a region from which $511~\mathrm{keV}$ gamma rays are detected. That means there are positrons, whose source could be as innocuous as the fluorine-18 used in PET scans. –  May 13 '13 at 15:42
  • Ok :) Got it :) – Ken May 13 '13 at 16:11

2 Answers2

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From the article to which you linked:

The cloud shines brightly in gamma rays due to a reaction governed by Einstein’s famous equation $E=mc^2$. Negatively charged subatomic particles known as electrons collide with their antimatter counterparts, positively charged positrons.

So you see, the very reason that we know it's a cloud of antimatter is precisely because it's already annihilating with normal matter that is floating in interstellar space. This won't create a chain reaction because there isn't that much more antimatter in our galaxy. (If there was, it would have annihilated a long time ago.)

And, as others have commented, the products of electron-positron annihilation (gamma rays) cannot go on to trigger further annihilations, so the idea of a chain reaction doesn't apply in this case.

Dmitry Brant
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    Then, similarly, that antimatter will annihilate with the first grain of interstellar dust that it encounters. – Dmitry Brant May 13 '13 at 15:12
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    It's been established that there can't be any chain reactions from antimatter (independent of its source), because the annihilation products will ultimately be photons. A large amount of antimatter is indeed dangerous to someone in the local vicinity, but space is big enough that the annihilation products don't pose a threat to someone far away. – Dmitry Brant May 13 '13 at 15:30
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Here are a few more points to add on to Dmitry's excellent answer.

A chain reaction requires amplification, in the sense that one reaction produces products that then initiate an average of $n$ further reactions, where $n>1$. For example, uranium fission produces neutrons, and if the piece of uranium is big enough and sufficiently enriched, those neutrons are capable of initiating further fissions with $n>1$.

The products of electron-positron annihilation are gamma rays, which don't initiate further reactions, i.e., $n=0$.

Empirically, our galaxy has existed for billions of years, and it hasn't undergone any cataclysm yet, nor do we observe any other galaxies in the universe that appear to be undergoing a cataclysm in the interstellar medium of the galactic core. (There are quasars, but their energy source is matter falling into the black hole at the core, not matter-antimatter annihilation in the interstellar medium.)