This is a very simple doubt that the title already explains: how to describe or narrate "force" damage? How to describe a eldrich blast for example? What is force damage? Could it be something like a bludgeoning? (I dont think so)

Typically it's thought of as raw magic or pure energy without an elemental type. Think of a bolt of fire or lightning but "neutral."

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

This is from the basic rules. 

I might narrate it as "raw magic" or "eldritch power", but just describing the look of the attack is probably sufficient, particularly for a spiritual weapon.

Describing as "raw magic" doesn't actually shows anything to the listener. We do not have previous experiences of how "raw magic" would work or how it could affect us so we could associate it with anything. Differently from "you take fire damage" - in this case the person would immediately imagine something burn, but "force"? raw magic? What does it mean?

I wish I could explain and detail the effects of it somehow

Most examples of force damage that have a defined physical effect turn out to be the equivalent of bludgeoning, piercing, or slashing damage done with a force object, so I'm not sure there's any real value to force continuing to exist as a distinct damage type (just call them magical physical damage -- very very few creatures have resistance to one but not the other), but it's been a distinct damage type since 3e (AD&D was less formal about its damage types).

Quote from Licozo >>

Describing as "raw magic" doesn't actually shows anything to the listener. We do not have previous experiences of how "raw magic" would work or how it could affect us so we could associate it with anything. Differently from "you take fire damage" - in this case the person would immediately imagine something burn, but "force"? raw magic? What does it mean?

I wish I could explain and detail the effects of it somehow

 The good news is, no matter what you pick, you won’t be wrong. 
Usually I leave it to the player to describe what their EB or spiritual weapon looks like. For an NPC, I would only really put much effort in if they were going to be a recurring foe. So “You take 8 force damage from the EB as pure magical energy rips into you.” Then leave it to the player’s imagination from there. 

I think of force damage as slightly transluent semi-solid magic energy matter. Think heat wave gut punching you.

I tend to describe it in the form of pure manifested energy, translucent, pulsating, unleashing itself as directed by the spell-caster. Sometimes coloured to provide additional flavour based on the caster. 

But I also tend to let characters personalise exactly how their magic manifests itself, and lean on them to provide descriptions for it, when it comes from them (especially the first couple of times they cast a spell, or in epic moments)

How would you describe a "force field" in science fiction? How does a "force" push work in star wars? Are you moving the entire body with your mind? If a telekinetic shove was strong enough to damage the creature/object would it do that by direct interaction with the matter of the object or would it be like hitting the object/creature with a hammer?

"Force" is something that we can't picture because there isn't an analog to anything representing the direct application of a force at a distance without an intervening method of application.

As a result, I tend to describe "force" damage as magic being manifested at the target, directly damaging the material structure of the creature or object. Depending on how you want to describe it, it could be a bolt/lance/ball of "force" that projects the damage directly to the structure of the creature/object. In the case of spiritual weapon, the magic assumes the shape of a weapon for the ease of identification by the caster and others but it isn't the "weapon" striking the target that does the damage, it is the magical projection of "force" directly to the target without an intervening mechanism that causes "force" damage.

In most cases, "force" damage appears to apply when magic does damage directly to the target without the magic being transformed into another type of damage first (eg elemental - fireball, cone of cold, firbolt, ray of frost, posion spray ... etc). "Force" damage is direct magical damage which is why it is so difficult to describe.

Mechanically, "force" damage exists just to have another damage category that doesn't overlap with the elemental (fire/cold/acid/etc), spiritual (radiant/necrotic) and weapon (b/p/s) damage categories - since there are a number of creatures immune or resistant to these damage types. Is it "necessary" to have this damage type? No - the game would probably be perfectly playable without damage types at all, but having force damage is a category for "everything else" that isn't already another type of damage.

I always think of the slow motion face punch or the cannonball to the gut (without the fist or cannonball).

I said that to a player " your whole body is jostled as if an invisible cannonball has struck you dead in the chest" and he recounted the old image of a carnival performer catching a cannonball in the gut.  Gif here... maybe...

If it doesn't have a visual component, I would describe it as a sudden, powerful concussive blast void of color or visual substance.

The players feel the force of the blow, knocking them backwards in a violent blast of energy. Their ears ring and their heads feel like they'll explode from a tremendous inner pressure. Disorientation takes over their thoughts as they try to bring the world back into focus.

It's not in the ruleset, but I usually storytell that the NPC or enemy briefly stumbles a step backwards, or maybe, double up clutching their gut and appearing winded. I'm not sure why not, but it does feel to me a little like force damage should maybe move the thing it hits. So I tend to house rule that if it's a crit, the enemy does indeed get moved somehow, I've had them knocked to their knees or maybe winded for a moment.

In any case, it's just about adding a bit of flavour and colour...nothing much more.

Quote from Aquilain >>

I'm not sure why not, but it does feel to me a little like force damage should maybe move the thing it hits.

 Probably because in the real world, that's exactly what force means - energy in the form of motion.

Describing as "raw magic" doesn't actually shows anything to the listener. We do not have previous experiences of how "raw magic" would work or how it could affect us so we could associate it with anything.

One thing you can do is focus on the form of the spell and maybe one aspect of it - warbling, pulsing, humming, etc. "Three bolts of pulsing magical energy" for example. This gives you some guidelines but allows the listeners to fill in the rest with their imagination - which honestly I prefer in cases like this. If the DM described every aspect of my spell, I would feel like they're taking over a bit of my character. This is full-on flavor territory, so you could describe magic missiles as anything from rifts in spacetime to rainbow-trailing nyan cats.

But if its really important to you that everyone feel like they're on the same page or the player is totally ambivalent to their spell descriptions, a quick image search of magic missile will give you many artists' renderings of what they envision the spell to look like.

Here's how I would describe it (starting at 3:30):

The magic these two are throwing at each other is concussive and invisible, something felt more than seen. Side note: I recall in the director's cut, Peter Jackson wanted this scene to play out as close to having two old men beating the stuffing out of each other as it could. I still get a kick out of that.

I think of it as atomic level damage. Force damage is not a physical force (bludgeoning and/or thunder cover that), it is damaging the target at some fundamental or atomic level.

If I had written the game, I would have called it somethng different, like "fundamental damage" or "raw damage". The word "force" isn't really right.

Then again, I would have written a lot of the damage types differently. For example, I'd remove thunder. There isn't any difference between thunder and bludgeoning. 

For the sound, I like to describe force as kind of like hard-light from the Halo games: "A high-pitched crackling sound as the bolt of energy zips past your head" or  "A dull thudding sound with an underlying fizzle, like a fire trying to keep itself alive"

As for what it feels like, I would once again go with physical damage with an underlying bit of elemental damage: "You feel a strange, burning sensation as the spear of energy passes through your torso, like there was a hand made of fire crushing your stomach, a sensation that almost makes you vomit" or "As the spectral blade slices through you, it feels more like your being hit with a warhammer, and that this hammer is hitting the inside of your body too, a new internal injury with every inch the blade moves"

I'd make force sound almost alien, like the things dealing force damage are things that should be breaking the laws of physics. Which does make sense, since the first thing I think of when I think of things that deal force damage is when a creature with incorporeal movement ends its turn inside an object.

Quote from Greenstone_Walker >>

I think of it as atomic level damage. Force damage is not a physical force (bludgeoning and/or thunder cover that), it is damaging the target at some fundamental or atomic level.

If I had written the game, I would have called it somethng different, like "fundamental damage" or "raw damage". The word "force" isn't really right.

Then again, I would have written a lot of the damage types differently. For example, I'd remove thunder. There isn't any difference between thunder and bludgeoning. 

 I'm gonna have to disagree with you on that. Thunder damage is sound, which I do agree could have been called something different, like "sonic". But being hit with a rock and being hit with a powerful sound wave are definitely two different things.

I agree with what one person said that it was damage on atomic level. the definition of atomic is relating to an atom or atoms. Lets say you are casting eldritch blast. So the way I like to describe it is that it basically hits the creature with so much force that without making any blood spill it will destroy the some of the atoms of where the creature was hit.

I think of it as a semi translucent, light purple, undulating, force. Think slime meets lightning.

Thats me personally 

From the Eldritch Blast description: "A beam of crackling energy streaks toward a creature within range"  - So I've always thought of it looking like whatever Emperor Palpatine is doing when he gets mad.....

Magic Missile, though, more like blaster shots from Star Wars :-)