Particles don't exist
You're familiar with atoms, and likely envision them as little balls. All of the stuffs of the world is made up of these atoms, bonded together. You probably know atoms are composed of protons, neutrons, and electrons - even smaller little balls.
Well even these sub-atomic particles are not actually little balls. They are not stuffs. There is no physical matter, according to (my understanding of) the "field" theories.
So physicists think that all of the universe is this single unified field of energy. It's just energy everywhere. They've also determined that "particles" like electrons can act as "particles" or as waves.
Well, the "particle" representation isn't really a tiny solid ball like I've long imagined. It is actually a concentration of energy in this unified field, and this energy is moving in particular patterns. It is that concentration and movement of energy that manifests as an electron.
You might think about an ocean. Let's say you get a small part of that ocean to spin, having its own current. The water is the unified field. The area that's spinning is a "particle". There is no new "stuff" there, but only a concentration and movement of water.
Because all things are just energy concentrating and moving, "particles" can be spontaneously created and destroyed. In particle accelerators, they make sub-atomic particles move near the speed of light, and they slam them into eachother. The initial particles are typically annihilated, and a slew of other particles are created, and decay, making other particles, and so-on. This is possible because none of the "particles" are actually stuff. They are just energy, and this energy moves into different forms - different concentrations and patterns of movement.
Or even more wild. Electrons are negatively charged. When you move two electrons near eachother, their negative charges cause them to repel eachother. WELL, apparently the mechanism of this repulsion is the exchange of photons. Photons shoot out of the electrons at eachother, which causes them to push apart from another.
Even MORE wild. In "empty" space, a vacuum, sub-atomic particles are constantly created and destroyed. Like a proton and anti-proton will both spontaneously come into existence, then re-combine and destroy eachother. (an anti-proton is like a proton but it has a negative charge).
This spontaneous creation of particles seems impossible when you think of the world as tiny bricks (or balls) stacked together to build bigger and bigger structures. But when everything is a field of energy, you can see that these "particles" are just this energy changing it's concentration and pattern of movement. It's still insane, but slightly less so.
WELL. On to the part I'm perhaps most fascinated by, is the idea that "Spacetime" is a singular unified thing. The idea that without "time" there is no 3 dimensional matter. That matter requires time in order to exist. Time is not some independent thing that just marches on while also a world of objects exist. There is one world of space & time and they are inherently linked.
To help you understand this, imagine you have a string (a couple feet long) with a single light on the end of it. You spin this light around in a circle. You spin it very very fast. Anybody looking at you spinning this light around will see a single solid circle (well, the outside-edge of it is solid, anyway). It is just one light, moving, and yet it is a solid circle.
Now take an "instant" of time. At any given "instant", there is only one light in one specific location. There is never an instant in which a whole circle is present. The presence of that circle requires the passage of time.
Like the light-on-a-string ... all stuffs of the universe are made from movement. Energy concentrates and moves, and it manifests as physical stuffs. Like the light moves and manifests as a circle.
And lastly ... I don't think any of this is proven as a matter of fact. The physicists have maths that work. They have equations they can use to determine what will happen in reality. The explanation of what is really the nature of reality (everything is a unified field of energy) is just an explanation, a theory of what the math means about the world.
For comparison, let's consider a ball being thrown. There's some math that says "If you throw the ball X hard in Y direction, then it will travel Z path". The ball will go up, curve, and come down. The math for this works.
Well, gravity essentially is pulling the ball down toward the center of the earth. But it could also be a different explanation. There could be tiny particles flying toward the center of the earth from space. Those particles could be pushing down on the top of the ball, causing it to move toward the ground. It could be this "push from above" or this "pull from below" and the math saying how the ball will move would be exactly the same. The movement of the ball is not proof that gravity exists as described. Gravity is just a theory here.
It is well-proven that "the ball will move in this way". But not well-proven that "the ball moves this way BECAUSE gravity pulls from below". I actually haven't read in detail about how gravity works, and it is possible that the "pulls from below" idea of gravity is well-proven (or entirely wrong). I don't really know. The point of the ball/gravity proposition was just to show that "the maths work" does not prove "this is the fundamental nature of reality".
Notes: My understanding is derived from the books What is Real? and The Tao of Physics. I haven't quite finished Tao of Physics yet, but I'm getting close. I am a layperson. I could be misunderstanding all of this. The ball/gravity example is my own. The light-on-a-string example is my own. The ocean/spin example is my own.
Update Mar 4, 2026: I finished the Tao of Physics. If anything, it strengthened what I was saying here, BUT there are other theories about the fundamental nature of reality that are different from the "field" theories I was primarily discussing above.