Permanent Matter?

According to Buddhism matter is like a wave:

• it arises
• it passes
• arises again
• passes again
• arises yet again
• passes yet again
• ...

While this happens there is also decay.

Impermanence here is that matter is not static. It arises and passes while decaying.

According to the law of Conservation of Energy, energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another. So the energy can be transformed into matter and vice versa. Also, we have a very limited understanding of dark matter and dark energy, we don't know as of now, what happens to the matter which goes into the black hole. So saying that 'matter remains constant' is incorrect.

Saying that;

...Dhammas are always changing, impermanent according to the Venerable Buddha?

You have to understand that every time the Buddha appears in the realm of Samsara the situation here is different, the audience of a Buddha will be different, what he will explain will be based on the time and circumstances, thus the Dhamma will be different and changing.

For an e.g. in the Heart Sutra the Buddha says to Ven Sariputta; >

... Nor is there pain or cause of pain,

Or cease in pain, or noble path To lead from pain; Not even wisdom to attain!

Attainment too is emptiness...

See the Buddha contradicted himself as now the audience is different.

As Dhammadhatu's answer pointed out, it's said that sankharas (not dhammas) are impermanent.

Three marks of existence

The three marks are:

• sabbe saṅkhārā aniccā — "all saṅkhāras (conditioned things) are impermanent"
• sabbe saṅkhārā dukkhā — "all saṅkhāras are unsatisfactory"
• sabbe dhammā anattā — "all dharmas (conditioned or unconditioned things) are not self"

The definition of "sankhara" is complicated -- see e.g. Can anyone explain Sanskara / Sankara indepth? -- but its meaning includes "a compound thing", "a thing that's put together composed of other things", "a thing that arises due to certain conditions".

An example is the human body -- made of different bits, happens due to certain conditions -- and is impermanent e.g. it lasts for a certain time and then disintegrates.

Sankharas can be mental too, i.e. things which -- having learned -- you put together or associate in your mind.

I don't know what physicists are saying these days (i.e. recently), about "matter in the universe" -- I think (i.e. what they say) that keeps changing too :-) -- have they theorised recently that there must be a lot of (unobservable?) dark matter (because otherwise their equations don't match their observations?)?

I don't think that has anything at all, really, to do with Buddhism though.

Isn't the amount of matter in the universe the same, no less, nor more, because nothing is ever lost, nor created, everything causes everything, and thus fixated and permanent?

If this is true, then how would that be possible if Dhammas are always changing, impermanent according to the Venerable Buddha?

All conditioned and/or compounded things are impermanent, in the sense that they would arise and cease, and also change. Only Nibbana is not impermanent. Everything else, including space, time, matter and energy are subject to arising, ceasing and changing.

Take gold for example. Gold atoms initially arose out of nuclear reactions in stars. Gold can be melted and molded using heat. Gold in its solid form can be dissolved in acid. This shows that gold is impermanent, arising, ceasing and changing. It is also compounded, in the sense that it is composed of subatomic particles like protons, electrons and neutrons. When gold is used to conduct electricity, it constantly replaces its electrons.

The total amount of matter is also constantly changing. Think about nuclear reactions in stars, radioactively decaying matter and nuclear reactors. Think of the burning of flammable objects. Think about decaying or digesting biological matter. Think about matter entering a black hole.

What about energy? Kinetic energy in a moving turbine of a windmill arose conditioned by another movement like wind. But then it ceases due to friction, and conditions the arising of other forms of energy like heat due to friction, and electricity due to electromagnetic induction. So, forms of energy too are impermanent, because they are arising, ceasing and changing. Forms of energy are also conditioned.

What about space? If you have 1 m³ of vacuumed empty space, is it permanent? If air or matter gets into it, then it's not empty anymore. According to quantum field theory, empty space is never truly empty, because quantum particles appear and disappear constantly. In cosmology, space in the universe is said to be expanding, causing galaxies to become further apart as time goes by.

What about time? Have you heard about time dilation? Time moves relatively a lot slower for an object that is moving closer to the speed of light. Therefore, time is conditioned.

The more we understand science, the more we understand that everything in the physical realm is impermanent, arising, ceasing and changing.

What about the laws of physics? Is that permanent? There is speculation among cosmologists that our universe may be one of many universes in a multiverse. They speculate that universes may be arising and ceasing like bubbles emerging and popping from a boiling soup. In each of these universes, the laws of physics may be different. So, even this could be impermanent.

Apart from Nibbana, it's almost impossible to find anything that is truly impermanent and unchanging for eternity.

From MN147 the Buddha tells Rahula:

“Everything that has a beginning has an end.”

If we are born, we must die.

Matter can be born from energy (e.g., the Big Bang) and matter can die into energy (e.g., nuclear explosion).