I thought scientists don’t actually know what the universe is made of, but matter and energy are two aspects of it. Matter can turn into energy and at least theoretically, vice versa.
We know pretty well what the universe is made of: matter and (probably) something else. We know a lot about the matter. It's certainly true that mass can decrease and some types of energy (which is more about why things happen than what they are made of) be released but that (perfectly reasonable) way of describing it as 'matter changes into energy' is very simplistic and doesn't really tell us much unless we can define what we mean by energy.
@serenada
Can you say some more on energy? I thought energy changed from one form to another - it cannot die as such.
That's certainly the way I prefer to teach it (which highlighting the limitations). The problem with it is that it leaves you open to 'well there might be unknown types of energy which butterflies use to sell furniture and you are just being closed minded'.
The Institute of Physics have successfully pushed to move teaching in the UK away from this model somewhat. I'm not a fan, because I think 'what is energy?' is one of the more complex questions in Physics and the 'energy transfer' model you describe works great in most situations and is easily understood.
To illustrate the problem:
Say you are next to a fire. You can detect the 'heat energy' and 'light energy' coming off the fire thanks to cells in your eyes and skin. Those two things really are part of the same phenomenon (Electromagnetic waves).
As that heat and light impacts on the surface of an object near the fire that object will heat up. The particules in it will vibrate faster. That's a completely different phenomenon. We can calculate the energy hitting the object and it will heat up by that much, but the only thing linking the two phenomena is the currency we use (Joules) to measure the energy involved. Really the heat energy travelling from the fire and the heat energy of the object are not the same thing at all.
If you bend that object you 'give it elastic potential energy', but really what you are doing is distorting the bonds between molecules to a higher energy state and when they snap back the object will transfer that energy to something else. If it hits your hand it will impart some heat energy, for example. Is that really an 'energy store' in the same way as when you store energy in food? Possibly, but what about when you store energy by raising the object in a gravity field, is that the same too?
Now the truth is that a lot of these phenomena are related because they are about how the matter of the universe behaves in relation to fundamental forces. You can describe how sound, elasticity, heat, movement all relate to each other closely enough to perhaps unfiy them under 'energy'- but does that make them the same as EM waves or nuclear binding energy.
My (very, very longwinded- apologies) point is that Energy as a thing in it's own right doesn't really exist. We give various phenomena the same units (joules) and use maths to work out how they affect each other, and we unify them all under the concept of energy because it's a really, really useful tool for working out how the world works.
But they are real, measurable phenomena governed by the way the physical properties of the universe- the model is always simpler than the reality. There can be no unknown mysterious 'vibrational energy' and you can't use it to send messages to some universal Father Christmas.