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Generally these theories make predictions about how the universe should be now. That is, they are testable. However, dark matter is based on the requirement to explain the universe as it is now based on other laws of physics remaining as we currently understand them. Perhaps that might be seen as a bit of 'god of the gaps', but it still makes a prediction that there is a form of matter that is dark and should, under some circumstances, but directly observable, we're just not in a technological position currently to do so, much like we weren't in a position to prove the Moon wasn't made of green cheese but could in the late 1960s (the USSR got a probe there first, but it probably wasn't in a position to determine the cheese/not cheese dichotomy but Apollo brought stuff back which is clearly not cheese).

Just because we don't know how right now how to test it does not mean it won't be testable. Many things have been postulated without the ability at that time to be tested but were later tested and some were falsified and some were not.

Not this nonsense again?

Some guesses are clearly wrong as they can be tested. Some guesses cannot be currently tested, but that doesn't mean they won't be tested in the future (see general relativity). So it's OK to speculate, as testing in the future may be possible, provided you aren't speculating something that by definition could never be tested. But pretty much all cosmologists are speculating on things that they expect to be testable at some point.

Challenge accepted.

There's a hotel in London called California. That can get confusing on many levels. I went once and obviously I am still here.

If you are thinking of going on a trip across the Sahara, you make sure you have a satellite phone. No one is going to be dotting the Sahara with cell towers because it would be uneconomic. Compared to the Sahara, deserts in the USA are tiny. Decent map projections make this pretty clear.

Colour me meh. In the UK it's 20%, although down from 30% a couple of decades ago. France is also at 20%. Only Germany really does well (5%).

What is the bar for "high-level reasoning"? Research on animals in recent years suggests they are capable of higher level reasoning than previously understood.

Well said. We're going to have to use pretty much everything that doesn't have severe consequences in and of itself.

Because of the fear of grid interruptions. The UK has plenty of renewables but there have been zero outages because of them, but a storm can bring down powerlines.

Most already use solar or wind power via... the grid. A few have local provision but more as a way of defraying costs by using existing roof space.

Indeed, but demand isn't constant and tends to be higher during the day. Given that nuclear is expensive, then sizing provision to meet the highest peak isn't really cost-effective. If you want expensive power that's more predictable (maintenance notwithstanding) then fine, but in general people would not be happy with the cost. France's power looks cheap, but it's partly hidden in taxes. But if people make a democratic decision to adopt nuclear entirely (it requires government subsidy) based on full information on true costs of the overall system of provision, then fair enough.

Indeed, this is why RCP8.5 was dropped by IPCC - CO2 emissions are less than this (the highest CO2 emissions scenario). People trumpeted it as the models being wrong, but it was removing a scenario. People don't always understand the distinction.

Firstly, they didn't - one of the potential circa 2 dozen scenarios included that but it wasn't a prediction. It's an important distinction. Secondly, it's not climate so whataboutism.