The single other particle in the universe. Therefore,

The law of conservation of linear momentum states that: The
total linear momentum of an isolated system remains constant in both magnitude
and direction. (Momentum and impulse booklet)

Outer space is not a perfect vacuum as it contains helium
and hydrogen atoms as well as cosmic dust (small particles of matter distributed
throughout space) even though they are in low density. (Unknown. (June 16, 2014). Is outer space a perfect vacuum? Retrieved
from http://tellmewhyfacts.com/2007/07/is-outer-space-perfect-vacuum.html.)
Thus, when an on object is moving in space it will experience friction.

Newton’s law of universal gravitation states that every
particle attracts every other particle in the universe with a force that is
directly proportional to the product of their masses and inversely proportional
to the square of the distance between their centres. (Newtons laws
booklet) Therefore, when an object is moving is space it will experience
the gravitational attractional forces toward every single other particle in the
universe.

Therefore, in outer space the forces that contribute to the
change of the momentum of an individual object will not only be the forces
acting between the objects themselves. As external forces such as friction and
gravity are acting upon the object causing momentum not to be conserved. (Isolated Systems. Retrieved from http://www.physicsclassroom.com/class/momentum/Lesson-2/Isolated-Systems)

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For the linear momentum of the system to be conserved the total
initial momentum of the system must be equal to the total final momentum of the
system. However, this will not occur in space as external forces are acting
upon the objects leading the final momentum of the system being less than that
of the total initial momentum of the system.

Therefore, it can be concluded that space is not an isolated
system and thus momentum will not be conserved in outer space. Thus, conservation of momentum
does not apply in outer space.