Riddle Poem

I am made up of everything,
But people know little about me
Nobody knows how I came to be,
or hardly anything aobut the things I contain inside me

I am dangerous and beautiful
I've puzzled mankind for centuries,
Like a well kept secret, I am unknown
I am ever changing

I am so vast, I have no end
I cannot be destroyed
I am everything
Undoubtabley, I am the world's greatest mystery