A dream that is not likely to ever come true is an example of something that would be described as pie in the sky.
pie in the sky
- a promise of benefits or a reward in an afterlife or in the remote future
- an unrealistically Utopian plan or project
1911, phrase originally in reference to the promises of religion taken from a song written by Joe Hill, "The Preacher and the Slave", a parody of the Salvation Army hymn "In the Sweet By and By". Hill was associated with the Industrial Workers of the World (commonly known as the Wobblies), who organized migrant laborers. When the workers returned to the cities, they faced the Salvation Army.
- You will eat, bye and bye,
- In that glorious land above the sky;
- Work and pray, live on hay,
- You'll get pie in the sky when you die