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"When a first body exerts a force F1 on a second body, the second body simultaneously exerts a force F2 = −F1 on the first body. This means that F1 and F2 are equal in magnitude and opposite in direction." - Newtons third law

Somewhat recent discoveries in science reveal that the universe is expanding. If Newtons third law holds true, what force is reacting against the expansion of the universe?

The expansion of the Universe is not a force.

but whatever made the universe expand in the first place is.

It is generally assumed that since a 'universe' is typically how we gather our information - such as : for the pursuit to the understanding of human existence , to have creative means to produce almost impossible challenges ; or to simply become a standing statistic. The actual error is creating a standard is that , it is completely possible the median has been simply recreated as a variable for observation by known prerequisites.

Dolphin42 wrote:The expansion of the Universe is not a force.

Dolphin42 is correct. Let me attempt to explain why.

Firstly no force need be applied for any currently ongoing expansion of something to exist. If I were to, say, kick a ball in an environment bereft of gravity it would move along the path upon which I set it. If some observer were later to come along and ask: "What is the third law reactive force to this moving ball?" the question would clearly be nonsensical. The ball is moving because a force was applied to it at some point in the past not because a force is currently impelling it.

Second: The universe is expanding uniformly, or at least near uniformly, in all directions. Think of the detonation of a bomb in empty space. A large quantity of energy is released from the chemical potential energy present in the bomb's pre-exploded state, the various parts of the bomb are then displaced in a more or less isotropic (uniformly spherical) fashion. The expansion of the universe which has been going on since the big bang is somewhat similar to this isotropic expansion.

Finally: Although the above discussions are helpful in demonstrating why there is no call for a 'reactionary force' to the universe's expansion they all deal with matter moving through space. In the case of the expanding universe it is the space itself which is stretching. Think of a polka dotted balloon. As you blow up the balloon each dot gets further apart from all the others simultaneously, but the dot itself is not moving across the balloon's surface, it is just that the surface itself is expanding. Thus, since F=ma, the concept of force in the traditional sense is not applicable since no mass is being accelerated through space, rather space itself is expanding. This is why some galaxies are moving away from us at speeds much greater than the speed of light, yet the universe's expansion does not violate relativity. Relativity states that no object can move through space faster than the speed of light in a vacuum, but places no upper bound on how quickly the space between two objects can expand.