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It is said that due to CMB radiation our universe has been expanding? Firstly what lead the universe to expand? What do scientists mean by dark energy? Will energy convert into matter?

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It is said that due to the CMB radiation our universe has been expanding?

No. It's the the other way round. The CMB is the result of the expansion of space. In the initial stages after the birth of the universe, there was hot plasma everywhere and light could not travel much farther. The universe was opaque at that time. But slowly as the Universe cooled and atoms began to form, light could travel farther. This light got redshifted due to the expansion of space and can be now detected as the CMB radiation. It's in the microwave region of the spectrum and just above absolute zero(about 2.7 K).

What do we mean by Dark Energy?

After Hubble's discovery, we came to the conclusion that the Universe is expanding. We expected the expansion to slow down as time progresses but what we observed as that the expansion was accelerating and is still doing so. Physicists and cosmologists did not have any explanations for this and so it was named "Dark Energy". Dark in the sense that it is unknown.

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Actually, the Universe's expansion is accelerating NOW. It was not always like this. We are now in a period of accelerated expansion. –  Py-ser Aug 7 at 2:42
    
I'm satisfied thank u sir –  user52076 Aug 8 at 13:42
    
Welcome! If you want to know more about modern cosmology, see this book and if you are looking for an easy to understand book see A Brief History of Time by Stephen Hawking. –  Yashbhatt Aug 8 at 15:00
    
@Py-ser What evidence do we have that the expansion was not accelerating just after the Big Bang? –  Yashbhatt Aug 8 at 15:01
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@user52076 If you are satisfied, please think about accepting the given answer. –  Arne Aug 10 at 7:20

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