Electromagnetic background radiation – as discovered in 1964, so ably recorded by COBE in the early nineties and more recently by ESA’s Planck microwave observatory.
The stuff that gives the universe an average (though lumpy) microwave “temperature” of just under 3 Kelvin. I know how the BBT suggests it got there. How else might it have come about?
Re: BBT itself, I am still not past figuring out how particles that emerged from an exploding singularity suddenly changed direction, in unlimited empty space, to start clumping together. So I have an open mind born of a recognition of ignorance.
