Raw Magic.

All the other kinds of magic are refined. Fire damage, ice damage, lightning damage, all that is magic refined into a specific form in order to produce a specific effect. Force damage is a much more crude version of that. If fire damage is a stone spear, ice damage is a bow and arrow, and lightning damage is a flint hammer, then force damage is saying "that's too much work" and just bludgeoning them to death with a decently resilient stick. Of course, you can still stab someone with a regular stick. You can throw your stabby stick at someone. You can hit someone with the blunt end of your stabby stick. It's just not quite as sophisticated as using the other methods. It's more simplistic.

Damage types don't represent the world on that level.

Damage types are artificial categories. Their function is to crudely divide the world of "every possible way to get hurt" into a dozen or so bins so that we can say what interacts with what else. (Force damage is the "interacts with nothing" bin.) But what hurts the creature isn't "force damage", it's whatever the spell does.

This is true of other damage types as well. Piercing damage is an abstraction; what hurt you is that you got stabbed with a knife. The main feature it has in common with other sources of piercing damage is that, in general, the same sort of armor will protect against all of them.

Asking what force damage looks like is like asking what an Aberration looks like. Go through the Monster Manual and look at everything with type Aberration. That's what it looks like.

From what I can tell, Force damage is the "None of the Above" option

And why, exactly, it's called "Force" Damage I couldn't tell you.

Basically, Force Damage deals any kind of damage that is not:

• Bludgeoning
• Piercing
• Slashing
• Fire
• Cold
• Acid
• Psychic
• Thunder
• Lightning
• Radiant
• Necrotic
• Poison

The most basic way to describe it is if the target was struck by Pure Entropy, like the Warp in the Warhammerverse.

For example, Disintegrate literally breaks an energy-entropy system (the target) and turns it to completely featureless ash.

However, that's not the best way to describe it, because saying something is "purely entropic" or "pure magical energy" basically means it works however you imagine it does when casting the spell.

Many people like to envision Eldritch Blast as a laser that shoots out and whacks someone (like Bludgeoning damage). However, it could also act like a beam that chars someone (like Fire), or transmutes some of their tissues into featureless ash like Disintegrate does.

It could even be something that removes so much luck that they immediately suffer a heart attack and die.

So, in essence, there's no real answer except this: Let the player choose what it looks like. You as a DM can handle the rest.

First, let's take a look in the definition of HP

Hit Points represent a combination of physical and mental durability, the will to live, and luck. Creatures with more Hit Points are more difficult to kill. Those with fewer Hit Points are more fragile.

So, a creature can be damaged not only by physical damage, but also by removing its will to live or even its luck.

Damage Types

As already mentioned, damage types have no actual meaning by themselves.

Different attacks, damaging spells, and other harmful effects deal different types of damage. Damage types have no rules of their own, but other rules, such as damage resistance, rely on the types.

This means your actual question and problem is: how do I imagine/interpret force damage?, as it has no actual mechanical meaning in the game itself.

Well, the truth is: there is no actual correct way to think about it, so you can think of it as you like. You could think of magic missile not actually hitting the body, but instead the "soul" of the target, simply reducing its will to live, although not actually doing any damage to the body (as a bludgeoning damage would). You can also think it as a pseudo-bludgeoning. Imagine it as it makes more sense to you. Note that the damage type does not have to be consistent on "how you imagine" it across spells - each spell has its own way to interact and damage the enemy creature. Again, damage types by themselves don't have an exact meaning.

For your bonus, Radiant damage actually is quite explicit on how it interacts with the creature.

Radiant: Radiant damage, dealt by a cleric’s Flame Strike spell or an angel’s smiting weapon, sears the flesh like fire and overloads the spirit with power.

Think telekinesis. It is effectively creating a pseudo-solid object out of magical energy and then using that to do damage. The examples given in the damage type description (magic missile and spiritual weapon) behave in this fashion.

I think you're looking for additional detail where none exists. Damage Types says...

Damage types have no rules of their own, but other rules, such as damage resistance, rely on the types.

More specifically, Force damage is described as...

Force is pure magical energy focused into a damaging form. Most effects that deal force damage are spells, including magic missile and spiritual weapon.

Until a source of damage interacts with something with resistance, immunity, vulnerability, or some other ability that cites an interaction with a type of damage, the damage type is functionally meaningless.

As strange as it may sound, fire damage doesn't burn things, cold damage doesn't freeze things, force damage doesn't have anything to do with mass or acceleration, etc. A specific spell or effect may say it sets things on fire, or freezes things solid, but that has no mechanical relation to the type of damage it may also deal: a spell could deal fire damage but freeze water, if that's what the spell says it does.

How you describe the results of a damage effect is purely a roleplaying or storytelling choice. Some spells and abilities provide guidance and descriptions, others do not.