Why is the universe expanding?

 Let me get to this question straight,

for this problem, we will have to divert from astrophysics to Mechanics,

Newton's first law stated that- every object will remain at rest or in uniform motion unless compelled to change its state by the action of an external force.

here on earth, we cannot see such phenomena because of many things like air resistance, friction, gravity, e.t.c but when it comes to space its nothing!!(literally)

no air, water gravity, friction,

here all the experiments can be done friction less !!!! (space)

Let's get back to the stars 

When big bang happened  it propelled many of its constituents with mind-boggling speeds,

and  due to their inertia and because space is kind of a frictionless plane they are still moving (till now!) because there is no external force to act upon them,

That's why the universe is expanding 

So till then

be curious, ask the right questions and think