I Does the amount of hidden mass remain constant?

[user-000]

According to the dark matter theories is the total percentage of the dark matter's mass across the Universe remains constant as time passes?

 

[Halc]

Dark energy has constant density while the density of matter (baryonic and dark) goes down, so on a percentage of mass/energy basis, the percentage of matter is going down.

Excluding dark energy, I suspect you're asking about the ratio of baryonic matter to dark matter (about 1:5 ratio). But either can fall into a black hole and cease to be distinct as either class. It's just a mass parameter then. Dark matter is also less likely to be captured by any small gravity well, so as for the percentage that remains outside the black holes, I would suspect that dark matter is gaining, however slowly.

Baryonic matter is consumed faster by stellar combustion, so there's another way that dark matter gains over time.

 

[ohwilleke]

According to the dark matter theories is the total percentage of the dark matter's mass across the Universe remains constant as time passes?

In the LambdaCDM "Standard Model of Cosmology" and most variants of it, the absolute amount of dark matter in the Universe (really, the "observable universe") remains constant for the purposes of the model, after it comes into being not long after the Big Bang, but the percentage of the total mass-energy of the Universe which is dark matter declines over time as the amount and proportion of the mass-energy of the Universe that is dark energy grows over time.

As a practical matter, the absolute amount of dark matter does not have to be exactly constant to make theories like LambaCDM work. It merely has to be approximately constant over a 13.5 billion year plus time period.

I say "approximately constant" because there are some means by which, for particular dark matter candidates, it can annihilate or decay, in addition to being sucked into black holes as mentioned in post #2 in this thread. Also, for dark matter candidates that are not thermal freeze out dark matter candidates, such as axion dark matter candidates, dark matter candidates can fit the model if they are created and destroyed on a regular basis in roughly equal amounts (or exactly equal amounts, depending upon the theory).

But any process that adjusts the amount of dark matter over time up or down is assumed to be statistically insignificant, relative to the total amount of dark matter in the universe, in all leading dark matter theories, when considering the uncertainties in our astronomy observations from which the proportion of the mass-energy of the universe that is dark matter is estimated.