Planets made of dark matter may have blown up, and we could see them

The story about what is inside a dark matter planet seems incorrect. It assumes an atmosphere of hydrogen upper layer and helium lower layer, with nothing inside. But, what is keeping the normal matter atmosphere "above ground" when that ground is dark matter that the atmosphere can only react with via gravity?

I would expect the atmosphere to go all the way to the center of the blob of dark matter, because there would be no force from the dark matter to repel it at the "surface", even if that dark matter is a "solid". And, with the normal matter gas being able to gravitationally act on itself, I would expect the "atmosphere" to continue to get more dense all the way to the center of the dark matter planet.

So, how do we know that the Sun isn't a blob of dark matter that attracted enough hydrogen to form a star? Or that the Earth is not a dark matter planet that attracted dust and made a rock?

Would we really be able to tell what the amount of dark matter is in the midst of normal matter?
 

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