Dark energy is a form of energy that appears to permeate all of space and exerts a sort of “repulsive” gravitational effect, which we are fairly certain is what is causing the universe’s expansion to accelerate.

The name Dark energy is mostly due to scientists having a very straightforward relationship with nomculture...

Observations of distant supernovae (Type Ia) in the late 1990s showed that the universe is not just expanding—it’s expanding at an increasing rate.

Dark energy, often modeled as having a constant energy density, effectively provides the appropriate pressure that pushes space apart to such an argument extent as what observations have been to date.

It has 'Negative Pressure' -

In general relativity, energy can exhibit “negative pressure.” This concept is abit counterintuitive because we normally associate mass-energy with a gravitational attraction.

At large scales it behaves or rather it is calculated to behave differently;

The Dark energy’s negative pressure counters gravity’s pull, accelerating the expansion rather than slowing it down.

Dominance in the Present Universe ( the most dominant interms of accounting for the mass)

dark energy is estimated to make up about 70% of the total energy budget of the universe, surpassing both dark matter (around 25%) and ordinary matter (about 5%).

Because it's density does not dilute as quickly as matter when the universe expands, dark energy is becoming increasingly significant over cosmic time.

Most probably it will increase in dominance over the hr upcoming Eons.

Hope this shed some light on the Dark

Best

H

By accelerating its expansion in a way that isn't captured purely by a positive cosmological constant. For the moment, however, the accelerating expansion of the Universe can be described by a positive cosmological constant, so dark energy is just another term for the contribution of the cosmological constant (that's as much a part of gravity as Newton's constant).

Maybe dark energy not only acts like repulsive gravity but IS repulsive gravity, and attractive gravity at the smaller scales is only empirical: https://www.linkedin.com/pulse/newton-einstein-concert-warren-frisina-x7t1e/?trackingId=41nzAD2VOso3Yij6O6p3Pw%3D%3D

Harri Shore,

there is no dark energy. The original paper (1998) has problems. I discussed it in my third book and concluded dark energy to be an illusion.

Other reports see no accelerating univese.

The idea of dark energy was introduced to explain the supernova type 1A brightness curve - supernovae at Z~1 were looking too dim. It was possible to explain this discrepancy in two different ways - the brightness is still follow the simple 1/r^2 rule but the space-time itself is changing too fast compare to previous thoughts (thus the necessity of dark energy pushing Universe apart) or the light is somehow additionally attenuated (light itself, nothing to do with space-time). Recent images of supernovae at much higher Z (higher than 2) clearly demonstrated the light scattering into the much larger than expected circle (well above the diffraction limit of the telescope, see the post here:

Tipikin: The higher Z, the stronger the effect of light scattering present in the supernova images. Supernova at Z=3.6 looks gigantic.

https://tipikin.blogspot.com/2025/01/the-higher-z-stronger-effect-of-light.html

It means that the same light is scattered into the much larger circle compare to the circle for smaller Z and therefore the supernova will be dimmer:

Tipikin: Light scattering observed on supernova type 1A - no "dark energy", the Hubble findings should be re-analysed.

https://tipikin.blogspot.com/2024/11/light-scattering-observed-on-supernova.html

It was not so easy to notice for Hubble telescope because:

1.Hubble has smaller mirror and less resolution - the diffraction circle is larger and small change of the diameter is not easy to notice (below error of measurements)

2.Hubble only measured supernovae at Z up to around 1 - the effect itself is much smaller.

Now much better telescope JWST easily see the supernovae at high Z (up to 3.6 so far and this is not limit!). The angular sizes of that supernovae are well above any reasonable measure of diffraction limit (even if the small trembling of telescope is assumed, because there are numerous photos with supernovae and presence of smaller angular size objects, so telescope is working perfectly):

Tipikin: Supernova at Z=2.83 - large angular size, smaller objects on the same image, relatively weak to completely exclude detector saturation - one more confirmation of light scattering

https://tipikin.blogspot.com/2024/10/supernova-at-z283-large-angular-size.html

In short - the original introduction of dark energy is due to the wrong interpretation of the experiment itself. Nothing to do with GR or any other theoretical ideas - simple error of experimentalists. It does not mean that there is completely absent something repulsive in the Universe, so the overall philosophical idea of "dark energy" may be still considered but it has no experimental proof. Recent DESI results confirms this - they have better fit if dark energy (in the present day interpretation) is omitted:

Tipikin: DESI and accelerated expansion of Universe - DESI results confirm the absence of accelerated expansion. Explanation - wrong interpretation of the results on supernovae.

https://tipikin.blogspot.com/2024/11/desi-and-accelerated-expansion-of.html

https://www.researchgate.net/publication/389143210_Pure_Time_Theory_-The_Illusion_of_Accelerated_Expansion_and_Its_Consequences_for_Galactic_Rotation