Why does the Universe need to have a beginning (or an end)? I am trying to understand why so many scientists believe in the Big Bang theory and why more people don't believe that the Universe has just always existed.

I think most scientists would reject an assumption in your question — that they believe the Universe needs to be one way or the other. Theories in science do not say how matters must be, they describe how they are.As to why scientists think the Universe does have a beginning, well,presumably it's because that hypothesis best fits the availableastronomical data. If you want the details, talk to yourlocal physicist.

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