How can we say that universe is expanding as we r sure about that
energy is neither created nor destroyed then from where does this more energy come from
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The theory of the expanding universe comes from Hubble.
Hubble studied galaxies and plotted their red shift vs. their estimated distance. From that he found that there seems to be a correlation between distance and redshift.
And since redshift is caused by something flying away from us, stronger redshift means its flying away faster. So since the further away a galaxy is, the stronger its redshift, implies that the further away something is from earth, the faster it flies away from us (thats where the Hubble constant comes from, it basically says how much the redshift increases if an object is a certain distance from earth).
From this the model formed, that the reason for them to be flying faster away from us, the further away they are, is because the universe is expanding. This model fits all of our current evidence and checks out with other implications as far as I know.
There are however other explentations on why stuff that is further away is more redshifted. At the moment the "Expanding Universe" model is the most accepted one in the community of scientists.