I am curious about physics and astrophysics in particular, although I have no academic basis.

Usually, I find it easy to understand the concepts and explanations, but I have particular difficulty in understanding the premises of why the CMB observation leads to the horizon problem.

In general, the explanations I see start from the premise that different areas of primordial space should have different temperatures, and, as light would not have had time to travel between all corners of space and homogenize temperature, the CMB should not be so uniform.

My problem is: where does the premise come from that primordial space should have different temperatures in each region?

My mental image is of a singularity that starts the big bang. This singularity, unless there is a reason to the contrary, should have given rise to a homogenous universe, even before particles would exist themselves.

If space starts to expand from a homogeneous environment, it should remain homogeneous until something happens to change that.

Therefore, for me, what would be expected is precisely that the CMB would be homogeneous, and not the other way around.

What am I missing? How we are certain that the universe “should” have been heterogeneous in temperature?