What do you call this thing in probability theory? I have studied it before but I forgot the name.
It is like when the possiblity of something happens is so small, but you created the experience so so many times, then the probability of that thing to happen is high.
It is like:
the possibility of having a life in a planet is $2\times10^{-17}\%$ but there are $5\times10^{19}$ planets, so the probablity of having a life becomes higher.
I know i did a wrong maybe example, but i am asking about the name of that field (or theory)
 A: Possibly you're thinking of the Poisson distribution.
Suppose you have a one-in-$1{,}000{,}000$ chance of success on each trial, and there are $3{,}600{,}000$ trials.  The expected number of successes is then $3.6$.  If we ask for the probability that there are exactly $5$ successes, we get
$$
\frac{3.6^5 e^{-3.6}}{5!} = \frac{3.6^5 e^{-3.6}}{120} \approx 0.13768. 
$$
Ladislaus Bortkiewicz's book The Law of Small Numbers, published in 1898, used the Poisson distribution to model the number of soldiers in the Prussian cavalry each year killed by being kicked by a horse. He also applied it to data on suicides by pre-pubescent children.
A: It seems like you may be referring to the Law of Large Numbers. The Wikipedia page linked to gives a good explanation, from what I can see. See also:


*

*the relevant part of this SE
answer for more information on the theorem, and why it is important,

*this SE question for another good, layman oriented explanation of the theorem,

*the top-rated answer to this SE
question, explaining away one person's confusion over the consequences of the theorem.
