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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?

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what antimatter in the center of our galaxy? The hypothrsis is for a blck hole, not antimatter. . 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
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! – Karthikeyan KC May 13 '13 at 14:31
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
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. – Chris White May 13 '13 at 15:42
up vote 7 down vote accepted

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.

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Thanks for the answer Dmitry :) But what if antimatter is being produced at the center, by some collisions like the one we do here at CERN or by beta decay! – Karthikeyan KC May 13 '13 at 15:03
Then, similarly, that antimatter will annihilate with the first grain of interstellar dust that it encounters. – Dmitry Brant May 13 '13 at 15:12
Will it now lead to a chain reaction as we have an antimatter source now? Assuming the matter is supplied from the stars and other objects, would it be catastrophic? Or on the other hand would it kill the source itself, stopping the collision of particles by annihilating with them? – Karthikeyan KC May 13 '13 at 15:19
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
:) Thank you Dmitry Brant :) – Karthikeyan KC May 13 '13 at 15:41

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.)

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