QUESTION:
Why do scientists believe in the Big Bang theory?

ANSWER from http://liftoff.msfc.nasa.gov/academy/universe/b_bang.html
The Big Bang Theory is the dominant scientific theory about the origin
of the universe. According to the big bang, the universe was created
sometime between 10 billion and 20 billion years ago from a cosmic
explosion that hurled matter and in all directions.

In 1927, the German priest Georges Lemaitre was the first to propose
that the universe began with the explosion of a primeval atom. His
proposal came after observing the red shift in distant nebulas by
astronomers to a model of the universe based on relativity. Years
later, Edwin Hubble found experimental evidence to help justify
Lemaitre's theory. He found that distant galaxies in every direction
are going away from us with speeds proportional to their distance.

The big bang was initially suggested because it explains why distant
galaxies are traveling away from us at great speeds. The theory also
predicts the existence of cosmic background radiation (the glow left
over from the explosion itself). The Big Bang Theory received its
strongest confirmation when this radiation was discovered in 1964 by
Arno Penzias and Robert Wilson, who later won the Nobel Prize for this
discovery.

Although the Big Bang Theory is widely accepted, it probably will
never be proved; consequentially, leaving a number of tough,
unanswered questions.