Now here comes the most controversial topic, contradicting most religious beliefs. Physicists used to believe universe was static (stable) based on their observations. Thus they concluded it was ethernal until a priest named George Lemade made a huge statement which wasn’t just based on his religious beliefs but after finding out observations done by Hubble and Lemade stated that Universe had origin as it keeps expanding.
So physicists started their quest to unravel the truth and assumed that since the Universe is expanding, there must be a point where everything was squeezed to the point where everything was a single point.( singularity). It’s just like an infinite number line which you can zoom in to see smallest units (km,meter, centimeter) and zoom out till infinity. But after the point of singularity they assumed that there was a big explosion which separated everything and that event is called Big Bang Theory.
But was it really a giant explosion, it was rather the stretching of space time which is still going on. It was a point of singularity which kept expanding and everything got separated. But how did the stretching took place??
Well it’s just my opinion, that’s where God comes up. It was God who made it happen.
Do we expand with Universe. No, its the space which is stretching. Imagine in a two dimensional world you and your friend are holding each other’s hands and the space below you starts stretching, will it separate you guys. Well it depends on how strongly you are holding each other’s hand, if you guys can withstand the friction force generated by the stretching ground. I’ll leave it up to you guys. But if you guys are far away from each other and you are holding a rope. Will you two get separated? Yes because the stretching rate is more than the force that is keeping you guys together.
Same is the case with solar system and other star systems, the gravity is strong enough to hold them together. But the bigger galaxies that are far away from each other there the gravitational pull is not enough to keep them hold them closer to each other. That’s why we observe red shift in space while observing stars indicating movement of stars away from us.