At about 1 second after the beginning of the universe, one type of particle was able to start traveling freely through the universe. These particles are still with us, but they are extremely hard to detect. They are called:

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The particles that were able to start traveling freely through the universe at about 1 second after the beginning of the universe are neutrinos. They are extremely hard to detect, but they are still present in the universe.

At About 1 Second After the Beginning of the Universe