We know that at the moment space is expanding and this means that in the past everything was close together, here the Big Bang theory has his origins.
We can see the moment when the universe was very packed together at the period of 1 second after the Big Bang and we can also here the echo of that period.
The problem begins when the universe is even younger then 1 second because the only vision of that period comes from our theories, due to the fact that we can't reproduce that period in a laboratory yet. The drawback of these theories is the fact that we have to use quantum (because of the small objects we have to deal with) and relativity (because of the massive objects that we have to deal with). However, those theories do not work together.
Two conversions of ripples in the radiation that have been converted to the frequencies that we can hear.